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)