crypto/internal/bigmod: don't panic on NewModulusFromBig(0)
Return an error instead. Makes usages of NewModulusFromBig a bit more
verbose, but better than returning nil or something and just moving the
panic down the road.
Fixes #60411
Change-Id: I10732c6ce56ccd9e4769281cea049dd4beb60a6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/498035
Auto-Submit: Roland Shoemaker <roland@golang.org> Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org> Reviewed-by: Damien Neil <dneil@google.com>