GrTessellator: add a null-check for a clusterfuzz test case.

BUG=695696

Change-Id: I1c001316721b3c042cff115c905ff0d2c2698ac0
Reviewed-on: https://skia-review.googlesource.com/9053
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
1 file changed