Fixed a NullPointerException case on Digest authentication.

Could occur when upgrading from a Debian package configured with Basic
authentication (as in release 1.92.9000) to a more recent one with
Digest authentication, without having re-encoded the admin password (for
example with dpkg-reconfigure).

As reported by eros on YaCy forum
(http://forum.yacy-websuche.de/viewtopic.php?f=23&t=5988#p33686).

source/net/yacy/http/YaCyLegacyCredential.java

@@ -65,8 +65,11 @@ public class YaCyLegacyCredential extends Credential {
     public boolean check(Object credentials) {
 
         if (credentials instanceof Credential) { // for DIGEST auth
-            return ((Credential) credentials).check(c);
-
+        	if(this.c == null) {
+        		/* credential may be null after switching from BASIC to DIGEST authentication without re-encoding the password */
+        		return false;
+        	}
+        	return ((Credential) credentials).check(this.c);
         }
         if (credentials instanceof String) { // for BASIC auth
             final String pw = (String) credentials;