avcodec/hevcdec: check that the local context list was allocated before dereferencing it

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.

Fixes ticket #9099.

Signed-off-by: James Almer <jamrial@gmail.com>