dcb.c

dcb_write, if write failed, gwbuf wasn't freed.
mysql_backend.c 
	gw_MySQLWrite_backend, if dcb wasn'r in POLLING state then write was skpiped but gwbuf wasn't freed.
mysql_client.c
	gw_MySQLWrite_client, disabled the use of dcb_write because it returns different values than the code it replaced. Also removed erroneous call of gwbuf_consume.
mysql_common.c
	mysql_send_custom_error, removed erroneous call of gwbuf_free.

server/modules/protocol/mysql_backend.c

@@ -472,6 +472,9 @@ gw_MySQLWrite_backend(DCB *dcb, GWBUF *queue)
          * Don't write to backend if backend_dcb is not in poll set anymore.
          */
         if (dcb->state != DCB_STATE_POLLING) {
+                /** Free buffer memory */
+                gwbuf_consume(queue, GWBUF_LENGTH(queue));
+                
                 skygw_log_write_flush(
                         LOGFILE_ERROR,
                         "%lu [gw_MySQLWrite_backend] Write to backend failed. "