gst/matroska/matroska-mux.c: Don't try to ready buffer duration from buffer that we don't own any  longer and that mi...

Original commit message from CVS:
Reviewed by: Tim-Philipp Müller  <tim at centricular dot net>
* gst/matroska/matroska-mux.c: (gst_matroska_mux_write_data):
Don't try to ready buffer duration from buffer that we don't
own any  longer and that might already have been unreffed.
(#321136)