fix for "negative seek offset" error during extension of heap files.

This would have always happend when a heap file exceeds 2GB.
should fix https://github.com/yacy/yacy_search_server/issues/372

source/net/yacy/kelondro/blob/Heap.java

@@ -117,7 +117,6 @@ public final class Heap extends HeapModifier implements BLOB {
         return super.size() + ((this.buffer == null) ? 0 : this.buffer.size());
     }
 
-    
     /**
      * test if a key is in the heap file. This does not need any IO, because it uses only the ram index
      * @param key
@@ -147,7 +146,7 @@ public final class Heap extends HeapModifier implements BLOB {
     private void add(byte[] key, final byte[] blob) throws IOException {
         assert blob.length > 0;
         if ((blob == null) || (blob.length == 0)) return;
-        final int pos = (int) this.file.length();
+        final long pos = this.file.length();
         try {
             this.index.put(key, pos);
             this.file.seek(pos);