Initialise CSecurityTLS::cert_cred properly

If we leave it at something random we'll get an invalid delete
if the handshake fails.
diff --git a/common/rfb/CSecurityTLS.cxx b/common/rfb/CSecurityTLS.cxx
index 9eeb76c..c90f49e 100644
--- a/common/rfb/CSecurityTLS.cxx
+++ b/common/rfb/CSecurityTLS.cxx
@@ -68,7 +68,7 @@
 static LogWriter vlog("TLS");
 
 CSecurityTLS::CSecurityTLS(bool _anon) : session(0), anon_cred(0),
-						 anon(_anon), fis(0), fos(0)
+						 cert_cred(0), anon(_anon), fis(0), fos(0)
 {
   cafile = X509CA.getData();
   crlfile = X509CRL.getData();