Fix potential overflow when calculating ImageSize am: 2cd3a5fbc1 am: 1101d6644e

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/giflib/+/29481377

Change-Id: I1b8cda6f000b47f06b3354e584c1f86d66e91f12
Signed-off-by: Automerger Merge Worker <[email protected]>