Check Number of MBs to Skip.

The number of skip mbs was parsed as 0 and impeg2d_dec_skip_mbs was getting
called with a large number because of underflow.
Added a check for the same

Bug: 63125953
Test: run PoC on ASAN-enabled mpeg2dec before/after
Change-Id: I07f43c1745e38e800751997e97d44d2bab0615a8
(cherry picked from commit 89b4c1cf9e2d18c27c2d9c8c7504e5e2d79ef289)