fix for http://forum.yacy-websuche.de/viewtopic.php?f=6&t=1351&p=9242#p9242

git-svn-id: https://svn.berlios.de/svnroot/repos/yacy/trunk@5154 6c8d7289-2bf4-0310-a012-ef5d649a1542

source/de/anomic/crawler/ResultURLs.java

@@ -284,9 +284,11 @@ public final class ResultURLs {
 
     public synchronized boolean remove(final String urlHash) {
         if (urlHash == null) return false;
+        String hash;
         for (int stack = 1; stack <= 6; stack++) {
             for (int i = getStackSize(stack) - 1; i >= 0; i--) {
-                if (getUrlHash(stack, i).equals(urlHash)) {
+                hash = getUrlHash(stack, i);
+                if (hash != null && hash.equals(urlHash)) {
                     removeStack(stack, i);
                     return true;
                 }