avcodec/utils: Only call codec->close if init has been called

avcodec/utils: Only call codec->close if init has been called

avcodec_open2() also called the AVCodec's close function if an error
happened before init had ever been called if the AVCodec has the
FF_CODEC_CAP_INIT_CLEANUP flag set. This is against the documentation of
said flag: "The codec allows calling the close function for deallocation
even if the init function returned a failure."

E.g. the SVQ3 decoder is not ready to be closed if init has never been
called.

Fixes: NULL dereference
Fixes: 25762/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SVQ3_fuzzer-5716279070294016
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>