fix fastcgi segfaults a better way

I think this is what julk originally wanted. He has a `m_keepConn` boolean that is set by `ConnectionFlags::KEEP_CONN` and if that is false, he calls this callback in both the error and the success case.

This doesn't segfault when the connection is abruptly closed by the requestor.

Closes #1522

Reviewed By: @scannell

Differential Revision: D1124995