avcodec/h264, videotoolbox: fix crash after VT decoder fails

The way videotoolbox hooks in as a hwaccel is pretty hacky. The VT decode
API is not invoked until end_frame(), so alloc_frame() returns a dummy
frame with a 1-byte buffer. When end_frame() is eventually called, the
dummy buffer is replaced with the actual decoded data from
VTDecompressionSessionDecodeFrame().

When the VT decoder fails, the frame returned to the h264 decoder from
alloc_frame() remains invalid and should not be used. Before
9747219958060d8c4f697df62e7f172c2a77e6c7, it was accidentally being
returned all the way up to the API user. After that commit, the dummy
frame was unref'd so the user received an error.

However, since that commit, VT hwaccel failures started causing random
segfaults in the h264 decoder. This happened more often on iOS where the
VT implementation is more likely to throw errors on bitstream anomolies.
A recent report of this issue can be see in
http://ffmpeg.org/pipermail/libav-user/2016-November/009831.html

The issue here is that the dummy frame is still referenced internally by the
h264 decoder, as part of the reflist and cur_pic_ptr. Deallocating the
frame causes assertions like this one to trip later on during decoding:

  Assertion h->cur_pic_ptr->f->buf[0] failed at src/libavcodec/h264_slice.c:1340

With this commit, we leave the dummy 1-byte frame intact, but avoid returning it
to the user.

This reverts commit 9747219958060d8c4f697df62e7f172c2a77e6c7.

Signed-off-by: wm4 <nfxjfg@googlemail.com>
