h264: fix the check for invalid SPS:num_ref_frames.

This patch set the limit to 16.

For information, thoses previous commits:
41f7e2d11d2dca23842ee89d530ca9fa15cec9d8
5cbb0e70a0a2ee99eb3cb09e837b9a1f7355b9bc
assumed it was either 30 or 32.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
