ossl.c: NULL check

* ext/openssl/ossl.c (ossl_make_error): check NULL for unknown
  error reasons with old OpenSSL, and insert a colon iff formatted
  message is not empty.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45271 b2dd03c8-39d4-4d8f-98ff-823fe69b080e