Don't crash on last_node

lib/tree_builder.py

@@ -181,11 +181,12 @@ class TreeBuilder:
             node = merge_before_last.merged
             last_node = None
             while node:
+                last_node = node
                 node = node.parent
                 if node and node.merged_into:
                     break
-                last_node = node
-            last_node.parent = None
+            if last_node:
+                last_node.parent = None
 
         if not last_merge.parent:
             last_merge.parent = last_merge.merged
@@ -196,6 +197,7 @@ class TreeBuilder:
         """Create a Factory tree (returning the top)"""
         factory_revisions = self.revisions_chain("openSUSE:Factory", package)
         self.add_merge_points(factory_revisions)
-        #factory_revisions.print()
+        # factory_revisions.print()
         self.prune_loose_end(factory_revisions)
+        # factory_revisions.print()
         return factory_revisions