rtpdtmfsrc: fix compiler warning

gstrtpdtmfsrc.c: In function 'gst_dtmf_src_prepare_message.isra.1':
gstrtpdtmfsrc.c:669:3: error: 's' may be used uninitialized in this function

gst/dtmf/gstrtpdtmfsrc.c

@@ -664,6 +664,8 @@ gst_dtmf_src_prepare_message (GstRTPDTMFSrc * dtmfsrc,
       break;
     case RTP_DTMF_EVENT_TYPE_PAUSE_TASK:
       return NULL;
+    default:
+      return NULL;
   }
 
   return gst_message_new_element (GST_OBJECT (dtmfsrc), s);