MDEV-30826 Invalid data on mysql.host segfaults the server after an upgrade to 10.4

convert empty host.db to "%", just as it's done for host.hostname
(in update_hostname())

mysql-test/main/grant5.result

@@ -453,4 +453,13 @@ insert mysql.host values (1);
 flush privileges;
 ERROR HY000: Fatal error: mysql.host table is damaged or in unsupported 3.20 format
 drop table mysql.host;
+#
+# MDEV-30826 Invalid data on mysql.host segfaults the server after an upgrade to 10.4
+#
+create table mysql.host (host char(60) binary default '' not null, db char(64) binary default '' not null, select_priv enum('n','y') collate utf8_general_ci default 'n' not null, insert_priv enum('n','y') collate utf8_general_ci default 'n' not null, update_priv enum('n','y') collate utf8_general_ci default 'n' not null, delete_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_priv enum('n','y') collate utf8_general_ci default 'n' not null, drop_priv enum('n','y') collate utf8_general_ci default 'n' not null, grant_priv enum('n','y') collate utf8_general_ci default 'n' not null, references_priv enum('n','y') collate utf8_general_ci default 'n' not null, index_priv enum('n','y') collate utf8_general_ci default 'n' not null, alter_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_tmp_table_priv enum('n','y') collate utf8_general_ci default 'n' not null, lock_tables_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_view_priv enum('n','y') collate utf8_general_ci default 'n' not null, show_view_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_routine_priv enum('n','y') collate utf8_general_ci default 'n' not null, alter_routine_priv enum('n','y') collate utf8_general_ci default 'n' not null, execute_priv enum('n','y') collate utf8_general_ci default 'n' not null, trigger_priv enum('n','y') collate utf8_general_ci default 'n' not null, primary key /*host*/ (host,db)) engine=myisam character set utf8 collate utf8_bin comment='host privileges;  merged with database privileges';
+insert mysql.host values('10.5.0.0/255.255.0.0','','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','N');
+flush privileges;
+drop table mysql.host;
+#
 # End of 10.4 tests
+#

mysql-test/main/grant5.test

@@ -408,4 +408,16 @@ insert mysql.host values (1);
 flush privileges;
 drop table mysql.host;
 
+--echo #
+--echo # MDEV-30826 Invalid data on mysql.host segfaults the server after an upgrade to 10.4
+--echo #
+
+# from mysql_system_tables.sql in 10.3:
+create table mysql.host (host char(60) binary default '' not null, db char(64) binary default '' not null, select_priv enum('n','y') collate utf8_general_ci default 'n' not null, insert_priv enum('n','y') collate utf8_general_ci default 'n' not null, update_priv enum('n','y') collate utf8_general_ci default 'n' not null, delete_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_priv enum('n','y') collate utf8_general_ci default 'n' not null, drop_priv enum('n','y') collate utf8_general_ci default 'n' not null, grant_priv enum('n','y') collate utf8_general_ci default 'n' not null, references_priv enum('n','y') collate utf8_general_ci default 'n' not null, index_priv enum('n','y') collate utf8_general_ci default 'n' not null, alter_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_tmp_table_priv enum('n','y') collate utf8_general_ci default 'n' not null, lock_tables_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_view_priv enum('n','y') collate utf8_general_ci default 'n' not null, show_view_priv enum('n','y') collate utf8_general_ci default 'n' not null, create_routine_priv enum('n','y') collate utf8_general_ci default 'n' not null, alter_routine_priv enum('n','y') collate utf8_general_ci default 'n' not null, execute_priv enum('n','y') collate utf8_general_ci default 'n' not null, trigger_priv enum('n','y') collate utf8_general_ci default 'n' not null, primary key /*host*/ (host,db)) engine=myisam character set utf8 collate utf8_bin comment='host privileges;  merged with database privileges';
+insert mysql.host values('10.5.0.0/255.255.0.0','','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','N');
+flush privileges;
+drop table mysql.host;
+
+--echo #
 --echo # End of 10.4 tests
+--echo #

sql/sql_acl.cc

@@ -2437,6 +2437,8 @@ static bool acl_load(THD *thd, const Grant_tables& tables)
                             "possible to remove this privilege using REVOKE.",
                             host.host.hostname, host.db);
       }
+      else if (!host.db)
+        host.db= const_cast<char*>(host_not_specified.str);
       host.access= host_table.get_access();
       host.access= fix_rights_for_db(host.access);
       host.sort= get_magic_sort("hd", host.host.hostname, host.db);
@@ -2445,8 +2447,7 @@ static bool acl_load(THD *thd, const Grant_tables& tables)
       {
         sql_print_warning("'host' entry '%s|%s' "
                         "ignored in --skip-name-resolve mode.",
-                         safe_str(host.host.hostname),
-                         safe_str(host.db));
+                         host.host.hostname, host.db);
         continue;
       }
 #ifndef TO_BE_REMOVED
@@ -3533,7 +3534,7 @@ ulong acl_get(const char *host, const char *ip,
     ACL_HOST *acl_host=dynamic_element(&acl_hosts,i,ACL_HOST*);
     if (compare_hostname(&acl_host->host,host,ip))
     {
-      if (!acl_host->db || !wild_compare(db,acl_host->db,db_is_pattern))
+      if (!wild_compare(db, acl_host->db, db_is_pattern))
       {
 	host_access=acl_host->access;		// Fully specified. Take it
 	break;