rtppassthroughpay: ensure buffer is writable before mapping writable

It is entirely possible that the incoming buffer into _chain() is not writable
and will result in a critical when trying to map().

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8131>

subprojects/gst-plugins-good/gst/rtp/gstrtppassthroughpay.c

@@ -23,12 +23,12 @@
  *
  * This elements pass RTP packets along unchanged and appear as a RTP
  * payloader element to the outside world.
- * 
+ *
  * This is useful, for example, in the case where you are receiving RTP
  * packets from a different source and want to serve them over RTSP. Since the
  * gst-rtsp-server library expect the element marked as `payX` to be a RTP
  * payloader element and assumes certain properties are available.
- * 
+ *
  * ## Example pipelines
  *
  * |[
@@ -419,6 +419,8 @@ gst_rtp_passthrough_pay_chain (GstPad * pad,
   GstRTPBuffer rtp_buffer = GST_RTP_BUFFER_INIT;
   guint pt, ssrc, seqnum, timestamp;
 
+  buffer = gst_buffer_make_writable (buffer);
+
   if (!gst_rtp_buffer_map (buffer, GST_MAP_READWRITE, &rtp_buffer)) {
     GST_ERROR_OBJECT (self, "Invalid RTP buffer");
     return gst_pad_push (self->srcpad, buffer);