avcodec/hevcdec: check that the local context list was allocated before dereferencing it
Since the decoder is not flagged as init cleanup capable, hevc_decode_free()
is being called manually if the hevc_decode_extradata() call fails at the end
of hevc_decode_init().
In a frame threading scenario, however, if AVCodec->init() returns an error,
ff_frame_thread_free() will be called regardless of the above flag being set
or not, resulting in hevc_decode_free() being called a second time for the
same context.
Workaround this by ensuring pointers are not dereferenced if they are NULL,
and set the decoder as init cleanup capable while at it.