MDEV-375 Server crashes in THD::print_aborted_warning with log_warnings > 3

Don't use ER(xxx) in THD::close_connection(), when current_thd is already reset to NULL.
Prefer ER_THD() or ER_DEFAULT() instead.

mysql-test/r/mdev375.result

+SET GLOBAL log_warnings=4;
+SET GLOBAL max_connections=2;
+SELECT 1;
+1
+1
+SELECT 2;
+2
+2
+ERROR HY000: Too many connections
+SELECT 0;
+0
+0
+SET GLOBAL log_warnings=default;
+SET GLOBAL max_connections=default;

mysql-test/t/mdev375.test

+#
+# MDEV-375 Server crashes in THD::print_aborted_warning with log_warnings > 3
+#
+SET GLOBAL log_warnings=4;
+SET GLOBAL max_connections=2;
+
+--connect (con1,localhost,root,,)
+SELECT 1;
+--connect (con2,localhost,root,,)
+SELECT 2;
+--disable_query_log
+--error ER_CON_COUNT_ERROR
+--connect (con3,localhost,root,,)
+--enable_query_log
+
+--connection default
+SELECT 0;
+
+SET GLOBAL log_warnings=default;
+SET GLOBAL max_connections=default;

sql/sql_class.h

@@ -3028,7 +3028,7 @@ class THD :public Statement,
     if (global_system_variables.log_warnings > threshold)
     {
       Security_context *sctx= &main_security_ctx;
-      sql_print_warning(ER(ER_NEW_ABORTING_CONNECTION),
+      sql_print_warning(ER_THD(this, ER_NEW_ABORTING_CONNECTION),
                         thread_id, (db ? db : "unconnected"),
                         sctx->user ? sctx->user : "unauthenticated",
                         sctx->host_or_ip, reason);