avformat/oggparseopus: Check opus_duration() return value

Regression since 39d11d599cd292485fe991cd22e10d7a1738b3bc

os->pduration would be wrongly assigned a negative value on invalid packets
instead of aborting.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
