detect if makeOffset failed

If we don't detect this, this imagefilter might not write to all of the pixels
it claims it will, resulting in uninitialized memory.

Bug: oss-fuzz:7209
Change-Id: I6c53757046b4d6051da1b317310c89b8ec4752ca
Reviewed-on: https://skia-review.googlesource.com/119145
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed