MDEV-8638: REVOKE ALL PRIVILEGES, GRANT OPTION FROM CURRENT_ROLE breaks replication

Fix the replication failure caused by incorect initialization of
THD::invoker_host && THD::invoker_user.

Breakdown of the failure is this:
Query_log_event::host and Query_log_event::user can have their
LEX_STRING's set to length 0, but the actual str member points to
garbage. Code afterwards copies Query_log_event::host and user to
THD::invoker_host and THD::invoker_user.

Calling code for these members expects both members to be initialized.
Eg. the str member be a NULL terminated string and length have
appropriate size.

mysql-test/suite/roles/rpl_grant_revoke_current_role-8638.result

+include/master-slave.inc
+[connection master]
+create role r1;
+set role r1;
+grant select on db.* to current_role;
+revoke all privileges, grant option from current_role;
+drop role r1;
+include/rpl_end.inc
+connection server_2;
+connection server_2;
+connection server_2;
+connection server_2;
+connection server_1;
+connection server_1;
+connection server_1;
+connection server_2;
+connection server_1;
+connection server_2;
+connection server_2;
+connection server_1;
+connection server_1;

mysql-test/suite/roles/rpl_grant_revoke_current_role-8638.test

+--source include/master-slave.inc
+--source include/have_binlog_format_mixed.inc
+
+--enable_connect_log
+
+create role r1;
+set role r1;
+grant select on db.* to current_role;
+revoke all privileges, grant option from current_role;
+drop role r1;
+
+--source include/rpl_end.inc

sql/log_event.cc

@@ -3631,10 +3631,25 @@ Query_log_event::Query_log_event(const char* buf, uint event_len,
   if (time_zone_len)
     copy_str_and_move(&time_zone_str, &start, time_zone_len);
 
-  if (user.length > 0)
+  if (user.length)
+  {
     copy_str_and_move((const char **)&(user.str), &start, user.length);
-  if (host.length > 0)
+  }
+  else
+  {
+    user.str= (char *) start++;
+    user.str[0]= '\0';
+  }
+
+  if (host.length)
+  {
     copy_str_and_move((const char **)&(host.str), &start, host.length);
+  }
+  else
+  {
+    host.str= (char *) start++;
+    host.str[0]= '\0';
+  }
 
   /**
     if time_zone_len or catalog_len are 0, then time_zone and catalog