h264: correct the check for invalid long term frame index in MMCO decode

The current check on MMCO parameters prohibits a "max long term frame index
plus 1" of 16 (frame idx of 15) for the "set max long term frame index" MMCO.
Fix this off-by-one error to allow the full range of legal values.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
