use safe increment for edge walker

Bug: oss-fuzz:6126
Change-Id: I8f6865cbbbfbe37acf940cc2d4ae93204ba7168d
Reviewed-on: https://skia-review.googlesource.com/107783
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed