The fact that crypto/ecdsa.Verify didn't reject negative inputs was a
mistake on my part: I had unsigned numbers on the brain. However, it
doesn't generally cause problems. (ModInverse results in zero, which
results in x being zero, which is rejected.)

The amd64 P-256 code will crash when given a large, negative input.

This fixes both crypto/ecdsa to reject these values and also the P-256
code to ignore the sign of inputs.

Change-Id: I6370ed7ca8125e53225866f55b616a4022b818f8
Reviewed-on: https://go-review.googlesource.com/22093
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>