[media] coda: fix timestamp list handling

Lock modification of the timestamp list with bitstream_mutex and do not
try to remove a timestamp element if the list is empty. This can happen
if the userspace feeds us garbage or multiple encoded frames in a single
buffer.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>