mpegvideo: operate with pointers to AVFrames instead of whole structs

The most interesting parts are initialization in ff_MPV_common_init() and
uninitialization in ff_MPV_common_end().

ff_mpeg_unref_picture and ff_thread_release_buffer have additional NULL
checks for Picture.f, because these functions can be called on
uninitialized or partially initialized Pictures.

NULL pointer checks are added to ff_thread_release_buffer() stub function.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
