fix writeImage when encode step fails, since data is null

Bug: skia:
Change-Id: Ifdfeba4b6d46a3169f60f3494245cf5abefe4c4a
Reviewed-on: https://skia-review.googlesource.com/113743
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
1 file changed