mod_ssl_ct: fix return values for custom extension callback

This is most likely a follow-up to r1628833.

At some point during the OpenSSL 1.0.2 beta, the contract for custom
extension callbacks changed from "returning -1 skips the extension" to
"returning -1 will issue a TLS fatal alert". This caused mod_ssl_ct to
abort TLS connections that it intended to ignore. Zero is the correct
return value for "do nothing" in 1.0.2.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1791845 13f79535-47bb-0310-9956-ffa450edef68