Public and private RSA keys have the same type in OpenSSL, so it's
probably prudent for us to catch this case with an error rather than
crash. (As we do if you, say, configure RSA-PSS parameters on an Ed25519
EVP_PKEY.) Bindings libraries, in particular, tend to hit this sort of
then when their callers do silly things.

Change-Id: I2555e9bfe716a9f15273abd887a8459c682432dd
Reviewed-on: https://boringssl-review.googlesource.com/17325
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>