bugfix: cast empty group

lualib/skynet.lua

@@ -178,7 +178,9 @@ end
 
 function skynet.cast(group, typename, ...)
 	local p = proto[typename]
-	return c.send(".cast", p.id, 0, mc(group, p.pack(...)))
+	if #group > 0 then
+		return c.send(".cast", p.id, 0, mc(group, p.pack(...)))
+	end
 end
 
 skynet.genid = assert(c.genid)

skynet-src/skynet_multicast.c

@@ -216,6 +216,8 @@ void
 skynet_multicast_cast(struct skynet_context * from, struct skynet_multicast_message *msg, const uint32_t *dests, int n) {
 	uint32_t source = skynet_context_handle(from);
 	skynet_multicast_copy(msg, n);
+	if (n == 0)
+		return;
 	int i;
 	int release = 0;
 	for (i=0;i<n;i++) {