Cleanup the packet pointer from packet node the moment we start using it

During flow queue flush, if an error occurs, pnode's packet pointer was
not cleaned up. Because of the multi stage flush logic we have now, a
subsequent flush will find this packet and try to flush it again, resulting
in potential double frees.

Closes BUG: #1401204

Change-Id: I848bbd62a53c129254eae65ab226cfb4e5baf2ff
Merge "Cleanup the packet pointer from packet node the moment we start using it"