tighten check when we might overflow legacy shader math

Bug: skia:7700
Change-Id: I2ca06b7ba36f2e17d6addd31b1f67a07d478e207
Reviewed-on: https://skia-review.googlesource.com/113264
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed