SkPerlinNoiseShader: handle pathological inputs

BUG=skia:6429
Change-Id: Ifc20c1f480c1396e40ab7fc3a3789aa7c2ae5d85
Reviewed-on: https://skia-review.googlesource.com/15896
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed