x509: fix handling of multiple URIs in Certificate#crl_uris

[rhenium]

The implementation of OpenSSL::X509::Certificate#crl_uris makes the assumption that each DistributionPoint in the CRL distribution points extension contains a single general name of type URI. This is not guaranteed by RFC 5280. A DistributionPoint may only contains something other than a URI, or more than one URI.

Let's include all URIs seen in the extension. If only non-URI pointers are found, return an empty array.

Fixes: https://github.com/ruby/openssl/issues/775

[rhenium]

cc: @bdewater, #275

This PR currently returns an empty array when the certificate includes the extension, but no URIs in it. Should it be nil in that case too?

[bdewater]

I think that'd be nice since an empty array is a bit useless otherwise.

[rhenium]

Fair enough. I changed it to return nil if the extension contains only non-URIs.