Decoder: Fix in checking first_mb_in_slice

Also, increment slice header only if previous slice had atleast one MB
This is to ensure there is no out of bound read for streams with 1 MB, and
due to error 2 slices were being accessed.

Bug: 33982658
Change-Id: I5f1918c09e922ca39f495f6059dfea3fa1d49448
(cherry picked from commit ef27433ca86c4084f0cff3e284f9e799c3fdfbec)
2 files changed