fix a potential overflow with MALLOC_LIMIT
fix a potential overflow with MALLOC_LIMIT

BUG=webp:321

Change-Id: Iab89dfe167fb394fcdffd3b2732d4ac9bef764b0

PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp