gif2webp: fix crash with NULL extension data
gif2webp: fix crash with NULL extension data

DGifGetExtension() may successfully return, but the data pointer should
still be validated

BUG=webp:310

Change-Id: I6cfe617871fef2fe07887e5f48bb20f7ab7cfb35

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