ParseOptionalChunks: clear int sanitizer warning
ParseOptionalChunks: clear int sanitizer warning

clears a warning of the form:
src/dec/webp_dec.c:182:62: runtime error: implicit conversion from type
'int' of value -2 (32-bit, signed) to type 'unsigned int' changed the
value to 4294967294 (32-bit, unsigned)

Bug: b/229626362
Change-Id: Ibf08f8bb9ec345381b51ca3872745368fca700c0

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