This serves as a basic sanity check and also prevents malformed private keys from setting P=0 and consuming large amounts of CPU and memory in the Exp call. Change-Id: Ife22069b989a7347d8deaaf13030df82ee59e87b Reviewed-on: https://go-review.googlesource.com/35246 Run-TryBot: Adam Langley <agl@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>