h264_parser: Fix POC parsing for the case where MMCO_RESET is absent.

The prev_ values were not set after parsing POC.
Increase length of the buffer decoded to parse enough safely.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
h264_parser: Fix POC parsing for the case where MMCO_RESET is present.

Signed-off-by: Anton Khirnov <anton@khirnov.net>