MMFMWK-8218 subparse: fix critical log print out when set rate <0 with external subtitle

When playback with external subtitle, subparse will force its own start
and stop position when seek. When rate < 0, critial log will print out
assertion 'stop != -1', so need avoid it when rate < 0.

upstream status: Pending
https://bugzilla.gnome.org/show_bug.cgi?id=771648
1 file changed