avcodec/av1dec: Fix leak in case of failure

avcodec/av1dec: Fix leak in case of failure

A reference to an AV1RawFrameHeader and consequently the
AV1RawFrameHeader itself and everything it has a reference to leak
if the hardware has no AV1 decoding capabilities or if some other error
happens. It happens e.g. in the cbs-av1-av1-1-b8-02-allintra FATE-test;
it has just been masked because the return value of ffmpeg (which
indicates failure when using Valgrind or ASAN) is ignored when doing
tests of type md5.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>