don't try to extract or blur a zero-sized bitmap

Bug:884473
Change-Id: Ic61238633f65c66db0d5d00c2c438f03552ec7c9
Reviewed-on: https://skia-review.googlesource.com/154634
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed