protect against fuzzer

Fuzzer data may generate path without a computable
length.

R=reed@google.com
Bug: 739190
Change-Id: I052540932937f24951ce66699080b8b959fb1a46
Reviewed-on: https://skia-review.googlesource.com/21500
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Cary Clark <caryclark@google.com>
1 file changed