Bug #17503460 MYSQL READ ONLY DOESN'T WORK FOR DROP TRIGGER

Problem: Drop Trigger succeeds even after setting read_only 
variable to ON.
Fix: Fix is to report the standard error 
(ER_OPTION_PREVENTS_STATEMENT)when global read_only variable 
is set to ON.

sql/sql_trigger.cc

@@ -458,6 +458,13 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
     */
     thd->lex->sql_command= backup.sql_command;
 
+    if (opt_readonly && !(thd->security_ctx->master_access & SUPER_ACL) &&
+        !thd->slave_thread)
+    {
+      my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0), "--read-only");
+      goto end;
+    }
+
     if (add_table_for_trigger(thd, thd->lex->spname, if_exists, & tables))
       goto end;