avcodec/decode: validate hw_frames_ctx when AVHWAccel.free_frame_priv is used

avcodec/decode: validate hw_frames_ctx when AVHWAccel.free_frame_priv is used

Validate that a hw_frames_ctx is available before using it for
the AVHWAccel.free_frame_priv callback, and don't require it to
be present when the callback is not in use by the HWAccel.

v2: check for free_frame_priv (Hendrik)
v3: return EINVAL (Christoph Reiter)
v4: better commit message (Hendrik)
v5: fix typo with missed frames_ctx (Lynne)

See[1]: https://github.com/msys2/MINGW-packages/pull/19050
Fixes: be07145109 ("avcodec: add AVHWAccel.free_frame_priv callback")
CC: Lynne <dev@lynne.ee>
CC: Christoph Reiter <reiter.christoph@gmail.com>
Signed-off-by: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Gbp-Pq: Name 0002-avcodec-decode-validate-hw_frames_ctx-when-AVHWAccel.patch