gst/multipart/multipartmux.c: Keep track of the buffer timestamp in the collectdata member instead of modifying the b...

Original commit message from CVS:
* gst/multipart/multipartmux.c: (gst_multipart_mux_compare_pads),
(gst_multipart_mux_queue_pads), (gst_multipart_mux_collected):
Keep track of the buffer timestamp in the collectdata member instead
of modifying the buffer without making the metadata writable first.
Fixes #382277.