decoder: Update check for first mb in slice

first_mb_in_slice shouldn't be >= mbs in the picture.

Test: poc in bugs
Bug: b/174238784
Bug: b/174507022
Bug: oss-fuzz:27856
Bug: oss-fuzz:28039
Change-Id: Id3a41c8c2ddf814910fc2d5dd4f57bdd84d28fec
(cherry picked from commit 7e06940dce7245f03fd950edf7f72ff321b2b451)
1 file changed