mms: fix assertion in input_stream_open

Hi,

upon trying to play an MMS stream added to the play list, I got this:

    mpd: /tmp/mpd/./src/input_stream.c:85: input_stream_open: Assertion `is->plugin->open == ((void *)0) || is->plugin == plugin' failed.

With the following patch applied, it works perfectly.

Thanks for having implemented MMS support :-).

Best regards,
Peter

src/input_mms.c

@@ -56,6 +56,7 @@ input_mms_open(struct input_stream *is, const char *url)
 	   decoder, which seems to work fine*/
 	is->mime = g_strdup("audio/x-ms-wma");
 
+	is->plugin = &input_plugin_mms;
 	is->data = m;
 	is->ready = true;
 	return true;