avcodec/h264, videotoolbox: do not return invalid frames on failure

If videotoolbox_common_end_frame failed, then the AVFrame was returned
to the API user with the dummy buffer (in AVFrame.buf[0]) still set, and
the decode call indicating success.

These "half-set" AVFrames with dummy buffer are a videotoolbox specific
hack, because the decoder requires an allocated AVFrame for its internal
logic. Videotoolbox on the other hand allocates its frame itself
internally, and outputs it only on end_frame. At this point, the dummy
buffer is replaced with the real frame (unless decoding fails).
