Fixed BUG #16175: Memory leak in rpl_trigger.test

  Allocating/freeing memory for the db member of THD
  is wholy managed by slave thread.

sql/sql_db.cc

@@ -1098,7 +1098,8 @@ err:
 bool mysql_change_db(THD *thd, const char *name, bool no_access_check)
 {
   int length, db_length;
-  char *dbname=my_strdup((char*) name,MYF(MY_WME));
+  char *dbname= thd->slave_thread ? (char *) name :
+                                    my_strdup((char *) name, MYF(MY_WME));
   char	path[FN_REFLEN];
   HA_CREATE_INFO create;
   bool system_db= 0;
@@ -1118,6 +1119,7 @@ bool mysql_change_db(THD *thd, const char *name, bool no_access_check)
       system_db= 1;
       goto end;
     }
+    if (!(thd->slave_thread))
       x_free(dbname);				/* purecov: inspected */
     my_message(ER_NO_DB_ERROR, ER(ER_NO_DB_ERROR),
                MYF(0));                         /* purecov: inspected */
@@ -1126,6 +1128,7 @@ bool mysql_change_db(THD *thd, const char *name, bool no_access_check)
   if (check_db_name(dbname))
   {
     my_error(ER_WRONG_DB_NAME, MYF(0), dbname);
+    if (!(thd->slave_thread))
       x_free(dbname);
     DBUG_RETURN(1);
   }
@@ -1156,6 +1159,7 @@ bool mysql_change_db(THD *thd, const char *name, bool no_access_check)
                dbname);
       mysql_log.write(thd, COM_INIT_DB, ER(ER_DBACCESS_DENIED_ERROR),
                       sctx->priv_user, sctx->priv_host, dbname);
+      if (!(thd->slave_thread))
         my_free(dbname,MYF(0));
       DBUG_RETURN(1);
     }
@@ -1168,6 +1172,7 @@ bool mysql_change_db(THD *thd, const char *name, bool no_access_check)
   if (my_access(path,F_OK))
   {
     my_error(ER_BAD_DB_ERROR, MYF(0), dbname);
+    if (!(thd->slave_thread))
       my_free(dbname,MYF(0));
     DBUG_RETURN(1);
   }