media: cedrus: hevc: Add check for invalid timestamp

Not all DPB entries will be used most of the time. Unused entries will
thus have invalid timestamps. They will produce negative buffer index
which is not specifically handled. This works just by chance in current
code. It will even produce bogus pointer, but since it's not used, it
won't do any harm.

Let's fix that brittle design by skipping writing DPB entry altogether
if timestamp is invalid.

Fixes: 86caab29 ("media: cedrus: Add HEVC/H.265 decoding support")
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>