Resolved an "attempt to index a nil value (local 'certs')" error in find_ciphers_group that caused false negatives in script output.

scripts/ssl-enum-ciphers.nse

@@ -673,7 +673,12 @@ local function find_ciphers_group(host, port, protocol, group, scores)
                 -- This may not always be the case, so
                 -- TODO: reorder certificates and validate entire chain
                 -- TODO: certificate validation (date, self-signed, etc)
-                local c, err = sslcert.parse_ssl_certificate(certs.certificates[1])
+                local c, err
+                if certs == nil then
+                  stdnse.debug1("Failed to retrieve certificate")
+                else
+                   c, err = sslcert.parse_ssl_certificate(certs.certificates[1])
+                end
                 if not c then
                   stdnse.debug1("Failed to parse certificate: %s", err)
                 elseif c.pubkey.type == kex.pubkey then