parseRSAPrivateKey calls rsa.PrivateKey.Precompute which triggers
divide-by-zero panic if either p or q is 1. Sanity check the parsed
values by calling rsa.PrivateKey.Validate.
Fixesgolang/go#11505
Change-Id: Ia6c9eccca0cfa49aaa58716e708c557a788bb204
Reviewed-on: https://go-review.googlesource.com/12356
Reviewed-by: Adam Langley <agl@golang.org>