SkPathOpsTSect: Avoid null dereference in asserts

We just move the asserts to after our null checks.

Change-Id: I96c02ff9cb60ca3a052198b30f205dc78899acc6
Reviewed-on: https://skia-review.googlesource.com/c/190564
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed