If there is a decoder initialization failure detected in avcodec_open2
after .init is called, allow graceful decoder .close to prevent leaking
libopus decoder allocations.

BUG=828526
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e43e97f0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>