crypto/internal/bigmod: fix panic [freeze exception]

[rolandshoemaker]

#60411 was only filed today, as it was blocked by an internal security investigation. The CL (http://go.dev/cl/498035) was sent & reviewed, but just slipped the deadline for the freeze. The change is small, tested, and reviewed, so I don't think it poses a significant risk.

cc @golang/release

[ianlancetaylor]

My understanding is that actual bug fixes (and doc changes) are OK at the moment. Freeze exceptions are required for new features.

(Please correct me if I am wrong.)

[rolandshoemaker]

Oh I completely misread the change in the freeze policy, and this makes complete sense 🤦, ignore me!