security patch for WeakPriorityBlockingQueue (produced a deadlock)

git-svn-id: https://svn.berlios.de/svnroot/repos/yacy/trunk@7307 6c8d7289-2bf4-0310-a012-ef5d649a1542

source/net/yacy/cora/storage/WeakPriorityBlockingQueue.java

@@ -198,7 +198,15 @@ public class WeakPriorityBlockingQueue<E> {
         }
         synchronized (this) {
             if (position >= this.queue.size() + this.drained.size()) return null; // we don't have that element
-            while (position >= this.drained.size()) this.poll();
+            Element<E> p;
+            int s;
+            while (position >= this.drained.size()) {
+                s = this.drained.size();
+                p = this.poll();
+                if (this.drained.size() <= s) break;
+                if (p == null) break;
+            }
+            if (position >= this.drained.size()) return null;
             return this.drained.get(position);
         }
     }