rtpbasepayload: map RTP buffer READWRITE when setting headers

GstRTPHeaderExtension::write can map the RTP buffer for reading. If that
happens on a buffer that is already mapped WRITE-only by the payloader,
the payloader's mapping gets invalidated (GstRTPBuffer::map will point
to a different instance of GstMemory).

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1173>

gst-libs/gst/rtp/gstrtpbasepayload.c

@@ -1711,7 +1711,7 @@ set_headers (GstBuffer ** buffer, guint idx, gpointer user_data)
   HeaderExt hdrext = { NULL, };
   GstRTPBuffer rtp = { NULL, };
 
-  if (!gst_rtp_buffer_map (*buffer, GST_MAP_WRITE, &rtp))
+  if (!gst_rtp_buffer_map (*buffer, GST_MAP_READWRITE, &rtp))
     goto map_failed;
 
   gst_rtp_buffer_set_ssrc (&rtp, data->ssrc);