MDEV-5308 Crash when running with slow_query_log=1

- Make log_slow_statement() always call delete_explain_query().

mysql-test/r/explain_slowquerylog.result

@@ -39,3 +39,15 @@ SELECT 'Server still alive?' as 'Yes';
 Yes
 Server still alive?
 DROP TABLE t1;
+#
+# MDEV-5308 Crash when running with slow_query_log=1
+#
+SET @save1= @@log_slow_rate_limit;
+SET @save2= @@long_query_time;
+SET log_slow_rate_limit=1000;
+SET long_query_time=0.000001;
+SELECT 1;
+1
+1
+SET log_slow_rate_limit=@save1;
+SET long_query_time=@save2;

mysql-test/t/explain_slowquerylog.test

@@ -47,3 +47,17 @@ SET max_join_size = 10;
 SELECT 'Server still alive?' as 'Yes';
 
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-5308 Crash when running with slow_query_log=1
+--echo #
+SET @save1= @@log_slow_rate_limit; 
+SET @save2= @@long_query_time; 
+
+SET log_slow_rate_limit=1000; 
+SET long_query_time=0.000001; 
+SELECT 1; 
+
+SET log_slow_rate_limit=@save1;
+SET long_query_time=@save2;
+

sql/sql_parse.cc

@@ -1554,7 +1554,7 @@ void log_slow_statement(THD *thd)
     statement in a trigger or stored function
   */
   if (unlikely(thd->in_sub_stmt))
-    DBUG_VOID_RETURN;                           // Don't set time for sub stmt
+    goto end;                           // Don't set time for sub stmt
 
 
   /* Follow the slow log filter configuration. */ 
@@ -1562,8 +1562,7 @@ void log_slow_statement(THD *thd)
       (thd->variables.log_slow_filter
         && !(thd->variables.log_slow_filter & thd->query_plan_flags)))
   {
-    delete_explain_query(thd->lex);
-    DBUG_VOID_RETURN; 
+    goto end; 
   }
  
   if (((thd->server_status & SERVER_QUERY_WAS_SLOW) ||
@@ -1580,7 +1579,7 @@ void log_slow_statement(THD *thd)
     */ 
     if (thd->variables.log_slow_rate_limit > 1 &&
         (global_query_id % thd->variables.log_slow_rate_limit) != 0)
-      DBUG_VOID_RETURN;
+      goto end;
 
     thd_proc_info(thd, "logging slow query");
     slow_log_print(thd, thd->query(), thd->query_length(), 
@@ -1588,6 +1587,7 @@ void log_slow_statement(THD *thd)
     thd_proc_info(thd, 0);
   }
 
+end:
   delete_explain_query(thd->lex);
   DBUG_VOID_RETURN;
 }