WebPFlipBuffer: fix integer overflow
WebPFlipBuffer: fix integer overflow

with large scale values the offset to the end of the buffer may exceed
32-bits range.

src/dec/buffer_dec.c:158:39: runtime error: signed integer overflow: 2 *
1275068416 cannot be represented in type 'int'
  #0 0x56444802bea5 in WebPFlipBuffer src/dec/buffer_dec.c:158:39

Bug: chromium:1196850
Change-Id: I08c8b69ada5d5dd3e9bf2b9006dffa0c5f2103a5

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