stifle /0 warnings in TF invert

No path through allows an infinite term in the inverse.
It's all funneled through tf_is_valid().

Bug: oss-fuzz:12430, oss-fuzz:12426
Change-Id: Id65cada5dceb7c18c1ff86ae29d012e86db98972
Reviewed-on: https://skia-review.googlesource.com/c/182072
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed