sasl: Fixed null pointer reference when decoding empty digest challenge

Fixed a null pointer reference when an empty challenge is passed to the
Curl_sasl_create_digest_md5_message() function.

Bug: http://sourceforge.net/p/curl/bugs/1193/
Reported by: Saran Neti

lib/curl_sasl.c

@@ -283,6 +283,9 @@ CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,
   if(result)
     return result;
 
+  if(!chlg)
+    return CURLE_LOGIN_DENIED;
+
   /* Retrieve nonce string from the challenge */
   if(!sasl_digest_get_key_value(chlg, "nonce=\"", nonce,
                                 sizeof(nonce), '\"')) {