h264_mp3toannexb_bsg: don't crash, but warn, if PPS/SPS not found.

Should an AVC-1 in MP4 stream not contain SPS or PPS NAL units,
this BSF is then unable to allocate an output buffer for the
modified stream. Warn that the resulting stream may be unplayable.

Fix roundup issue #2386.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
