Check return of _load_state

In error scenarios there might be no loadable state available.
This patch adds some robustness handling this scenarios
by checking the result and possible bailing out.

Change-Id: I02e2731b14bec22c22db08d60d8d15db1911a84e
Signed-off-by: Andreas Florath <andreas@florath.net>

diskimage_builder/block_device/blockdevice.py

@@ -179,6 +179,8 @@ local_loop:
         """Cleanup all remaining relicts - in good case"""
 
         dg, reverse_order, state = self._load_state()
+        if dg is None:
+            return 0
         for node in reverse_order:
             node.cleanup(state)
 
@@ -191,6 +193,8 @@ local_loop:
         """Cleanup all remaining relicts - in case of an error"""
 
         dg, reverse_order, state = self._load_state()
+        if dg is None:
+            return 0
         for node in reverse_order:
             node.delete(state)