crypto: check key type in GetPeerCertificate()

Works around the following exception:

  Error: 140463203215168:error:0607907F:digital envelope
  routines:EVP_PKEY_get1_RSA:expecting an rsa key:
  ../deps/openssl/openssl/crypto/evp/p_lib.c:288:
    at CleartextStream._pusher (tls.js:656:24)
    at SlabBuffer.use (tls.js:199:18)
    at CleartextStream.CryptoStream._push (tls.js:483:33)
    at SecurePair.cycle (tls.js:880:20)
    <snip>

The issue has been solved properly in v0.10 and the master branch as of
commit c6e2db2 ("crypto: clear error stack"). This is the "back-port"
to v0.8.

For some (rather unquantifiable) reason the original fix only works for
the tls module in v0.8 but not the https module unless OpenSSL is
downgraded to 0.9.8. Upgrading OpenSSL does *not* fix it, however.

The https module doesn't appear to be at fault; upgrading it to v0.10
doesn't fix the issue. That leaves either the tls or the http module
(that https derives from) but the changes to those modules are too
massive to back-port as-is.

`git bisect` over the v0.8 -> v0.10 commits didn't show up anything
useful, it pinpoints c6e2db2 as the commit that fixes things.

I've spent several hours on this now and seeing that v0.8 is in
maintenance mode, this cheap hack will have to do.

Fixes #4771.