remove two ubsan warnings
remove two ubsan warnings

(regarding uint overflow)

Change-Id: I1a76e4b1268370b6b7d6a1aa93b99e57f55fd02e

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