crash rather than overflow in SkTDArray

This adds explicit overflow checks to the two likeliest
places where non-buggy code could overflow SkTDArray.

We have an #ifdef'd out PathMeasure_explosion GM that
overflows before this CL and aborts with it.

Bug: skia:7674

Change-Id: Ia0c430f4a8bb9bad687d13c875f604fd7da45aab
Reviewed-on: https://skia-review.googlesource.com/122342
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed