previously calls to WebPPictureCopy() weren't checked, causing a crash
when the canvas copies were accessed later in the loop
Tested:
for i in `seq 1 1125`; do
export MALLOC_FAIL_AT=$i
./examples/gif2webp gif_file
./examples/gif2webp -mixed gif_file
done
Change-Id: I186d13be0ec8f3b72b7d247a8482590c1b1be541
e471728754
