Check for valid normals when offsetting

Bug: oss-fuzz:11546
Change-Id: I45cc443bcee240eebfa72fb0bdb19cb43192804f
Reviewed-on: https://skia-review.googlesource.com/c/173422
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed