MDEV-4462 mysqld gets SIGFPE when mysql.user table is empty

avoid divison by zero

mysql-test/r/empty_user_table.result

+create table t1 as select * from mysql.user;
+truncate table mysql.user;
+flush privileges;
+connect(localhost,u1,,test,MASTER_PORT,MASTER_SOCKET);
+ERROR 28000: Access denied for user 'u1'@'localhost' (using password: NO)
+insert mysql.user select * from t1;
+drop table t1;
+flush privileges;

mysql-test/t/empty_user_table.test

+#
+# MDEV-4462 mysqld gets SIGFPE when mysql.user table is empty
+#
+
+source include/not_embedded.inc;
+
+create table t1 as select * from mysql.user;
+truncate table mysql.user;
+flush privileges;
+
+--replace_result $MASTER_MYSOCK MASTER_SOCKET $MASTER_MYPORT MASTER_PORT
+--error ER_ACCESS_DENIED_ERROR
+connect (fail,localhost,u1);
+
+insert mysql.user select * from t1;
+drop table t1;
+flush privileges;
+

sql/sql_acl.cc

@@ -7367,6 +7367,12 @@ static bool find_mpvio_user(MPVIO_EXT *mpvio, Security_context *sctx)
     cs->coll->hash_sort(cs, (uchar*) sctx->user, strlen(sctx->user), &nr1, &nr2);
 
     pthread_mutex_lock(&acl_cache->lock);
+    if (!acl_users.elements)
+    {
+      pthread_mutex_unlock(&acl_cache->lock);
+      login_failed_error(mpvio->thd);
+      return 1;
+    }
     uint i= nr1 % acl_users.elements;
     ACL_USER *acl_user_tmp= dynamic_element(&acl_users, i, ACL_USER*);
     mpvio->acl_user= acl_user_tmp->copy(mpvio->thd->mem_root);