xen/control: short circuit xctrl_on_watch_event on spurious event

If there's no data to read from xenstore short-circuit
xctrl_on_watch_event to return early, there's no reason to continue
since the lack of data would prevent matching against any known event
type.

Sponsored by:	Citrix Systems R&D
MFC with:	r352925
MFC after:	1 week

sys/dev/xen/control/control.c

@@ -360,7 +360,7 @@ xctrl_on_watch_event(struct xs_watch *watch, const char **vec, unsigned int len)
 	
 	error = xs_read(XST_NIL, "control", "shutdown",
 			&result_len, (void **)&result);
-	if (error != 0)
+	if (error != 0 || result_len == 0)
 		return;
 
 	/* Acknowledge the request by writing back an empty string. */