MDEV-23328 Server hang due to Galera lock conflict resolution

* Fix error handling NULL-pointer reference
* Add mtr-suppression on galera_ssl_upgrade

mysql-test/suite/galera/r/galera_ssl_upgrade.result

 connection node_1;
-call mtr.add_suppression("WSREP: write_handler(): protocol is shutdown.*");
+call mtr.add_suppression("WSREP: write_handler().*");
 connection node_2;
-call mtr.add_suppression("WSREP: write_handler(): protocol is shutdown.*");
+call mtr.add_suppression("WSREP: write_handler():.*");
 connection node_1;
 connection node_2;
 connection node_1;
@@ -24,5 +24,7 @@ connection node_1;
 SELECT VARIABLE_VALUE = 2 FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_NAME = 'wsrep_cluster_size';
 VARIABLE_VALUE = 2
 1
-disconnect node_2;
-disconnect node_1;
+connection node_1;
+call mtr.add_suppression("WSREP: write_handler().*");
+connection node_2;
+call mtr.add_suppression("WSREP: write_handler():.*");

mysql-test/suite/galera/t/galera_ssl_upgrade.test

@@ -9,9 +9,9 @@
 --source include/have_ssl_communication.inc
 
 --connection node_1
-call mtr.add_suppression("WSREP: write_handler(): protocol is shutdown.*");
+call mtr.add_suppression("WSREP: write_handler().*");
 --connection node_2
-call mtr.add_suppression("WSREP: write_handler(): protocol is shutdown.*");
+call mtr.add_suppression("WSREP: write_handler():.*");
 
 # Save original auto_increment_offset values.
 --let $node_1=node_1
@@ -72,4 +72,8 @@ SELECT VARIABLE_VALUE = 2 FROM INFORMATION_SCHEMA.GLOBAL_STATUS WHERE VARIABLE_N
 # Restore original auto_increment_offset values.
 --source include/auto_increment_offset_restore.inc
 
---source include/galera_end.inc
+--connection node_1
+call mtr.add_suppression("WSREP: write_handler().*");
+--connection node_2
+call mtr.add_suppression("WSREP: write_handler():.*");
+

sql/sql_parse.cc

@@ -9093,7 +9093,7 @@ void sql_kill(THD *thd, longlong id, killed_state state, killed_type type)
 #ifdef WITH_WSREP
   return;
  wsrep_error_label:
-  my_error(ER_CANNOT_USER, MYF(0), wsrep_thd_query(thd));
+  my_error(ER_CANNOT_USER, MYF(0), " ");
 #endif /* WITH_WSREP */
 }
 
@@ -9128,7 +9128,7 @@ void sql_kill_user(THD *thd, LEX_USER *user, killed_state state)
 #ifdef WITH_WSREP
   return;
  wsrep_error_label:
-  my_error(ER_CANNOT_USER, MYF(0), user->user.str);
+  my_error(ER_CANNOT_USER, MYF(0), " ");
 #endif /* WITH_WSREP */
 }