vp9: correct context buffer resize check

allocations within vp9_alloc_context_buffers() rely on mi_rows/mi_cols
individually, use those to determine whether to realloc rather than
stride and stride * rows. this fixes a crash with some fuzzed files for
invalid accesses into last_frame_seg_map and above_context.

Change-Id: I7b9f40dcf170d443890f3bd2acd285507943c7d4
4 files changed