Previously it was possible to craft a DSA private key that would cause
Sign() to loop forever because no signature could be valid. This change
does some basic sanity checks and ensures that Sign will always
terminate.

Thanks to Yolan Romailler for highing this.

Be aware, however, that it's still possible for an attacker to simply
craft a private key with enormous values and thus cause Sign to take an
arbitrary amount of time.

Change-Id: Icd53939e511eef513a4977305dd9015d9436d0ce
Reviewed-on: https://go-review.googlesource.com/33725Reviewed-by: Yolan Romailler <y@romailler.ch>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>