Only emit RSA_R_BAD_VERSION on bad RSAPrivateKey versions.
I was a little bit too lazy in error handling here.
Change-Id: I9954957d41d610e715c1976a921dedeb8cb49d40
Reviewed-on: https://boringssl-review.googlesource.com/6240
Reviewed-by: Adam Langley <alangley@gmail.com>