fix dashimpl underflow

Previous impl would assert (and read past legal memory) for the new test.

Bug: skia: 8274
Bug: 875494
Change-Id: I2a2e20085d54d611151a9e20ae9cebf33c511329
Reviewed-on: https://skia-review.googlesource.com/148940
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed