rtpptdemux: just drop invalid rtp packets instead of erroring out

Apparently linphone sends an invalid RTP packet as very
first packet. We want to ignore that instead of erroring
out (same for any other invalid packets really).

https://bugzilla.gnome.org/show_bug.cgi?id=741398

gst/rtpmanager/gstrtpptdemux.c

@@ -448,8 +448,8 @@ gst_rtp_pt_demux_chain (GstPad * pad, GstObject * parent, GstBuffer * buf)
   /* ERRORS */
 invalid_buffer:
   {
-    /* this is fatal and should be filtered earlier */
-    GST_ELEMENT_ERROR (rtpdemux, STREAM, DECODE, (NULL),
+    /* this should not be fatal */
+    GST_ELEMENT_WARNING (rtpdemux, STREAM, DEMUX, (NULL),
         ("Dropping invalid RTP payload"));
     gst_buffer_unref (buf);
     return GST_FLOW_ERROR;