Fix NullPointerException in NTLM auth

Author: dwickern

client/src/main/java/org/asynchttpclient/ntlm/NtlmEngine.java

@@ -150,11 +150,17 @@ private static String stripDotSuffix(final String value) {
 
     /** Convert host to standard form */
     private static String convertHost(final String host) {
+        if (host == null) {
+            return null;
+        }
         return stripDotSuffix(host).toUpperCase();
     }
 
     /** Convert domain to standard form */
     private static String convertDomain(final String domain) {
+        if (domain == null) {
+            return null;
+        }
         return stripDotSuffix(domain).toUpperCase();
     }