Don't crash on packages that didn't get updates in devel

lib/tree_builder.py

@@ -175,16 +175,17 @@ class TreeBuilder:
                 merge_before_last = last_merge
                 last_merge = factory_node
             factory_node = factory_node.parent
-        # we need to find the last merged_into that didn't end nowhere
-        # and cut the rope there
-        node = merge_before_last.merged
-        last_node = None
-        while node:
-            node = node.parent
-            if node and node.merged_into:
-                break
-            last_node = node
-        last_node.parent = None
+        if merge_before_last:
+            # we need to find the last merged_into that didn't end nowhere
+            # and cut the rope there
+            node = merge_before_last.merged
+            last_node = None
+            while node:
+                node = node.parent
+                if node and node.merged_into:
+                    break
+                last_node = node
+            last_node.parent = None
 
         if not last_merge.parent:
             last_merge.parent = last_merge.merged
@@ -195,5 +196,6 @@ 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()
         self.prune_loose_end(factory_revisions)
         return factory_revisions