Fix segfault in audit plugin codepath

Summary:
The THD pointer is not always available when the audit plugin is called. The causes a problem when we try to access thd->query_id.

 Fix by doing a null check first. If the THD is null, then we copy what is done for thread_id, we just set query_id to 0.

Squash with: 1def6b7 Expose more information to audit plugin

Test Plan: mtr

Reviewers: jkedgar, hermanlee4, drilibo

Reviewed By: drilibo

Subscribers: webscalesql-eng

Differential Revision: https://reviews.facebook.net/D58287

sql/sql_audit.cc

@@ -92,7 +92,7 @@ static void general_class_handler(THD *thd, uint event_subtype, va_list ap)
   event.database_length= va_arg(ap, unsigned int);
   event.query_attributes= va_arg(ap, const char*);
   event.query_attributes_length= va_arg(ap, unsigned int);
-  event.query_id= (unsigned long long) thd->query_id;
+  event.query_id= thd ? (unsigned long long) thd->query_id : 0LL;
   event_class_dispatch(thd, MYSQL_AUDIT_GENERAL_CLASS, &event);
 }