Correction of fix for bug #12280 "Triggers: crash if flush tables".

We should not assume that "thd" argument of reload_acl_and_cache() is
non-zero. Failure to do so will cause server to crash when one sends
SIGHUP to it.


sql/sql_parse.cc:
  reload_acl_and_cache():
    When we call this function from SIGHUP handler we pass 0 as "thd" argument
    to it. So we should not assume that thd is non-zero in it.

sql/sql_parse.cc

@@ -6517,7 +6517,7 @@ void add_join_natural(TABLE_LIST *a, TABLE_LIST *b, List<String> *using_fields)
 
   SYNOPSIS
     reload_acl_and_cache()
-    thd			Thread handler
+    thd			Thread handler (can be NULL!)
     options             What should be reset/reloaded (tables, privileges,
     slave...)
     tables              Tables to flush (if any)
@@ -6539,7 +6539,7 @@ bool reload_acl_and_cache(THD *thd, ulong options, TABLE_LIST *tables,
   select_errors=0;				/* Write if more errors */
   bool tmp_write_to_binlog= 1;
 
-  if (thd->in_sub_stmt)
+  if (thd && thd->in_sub_stmt)
   {
     my_error(ER_STMT_NOT_ALLOWED_IN_SF_OR_TRG, MYF(0), "FLUSH");
     return 1;