Fix bug in BgpXmppChannel::FlushDeferQ.

https://bugs.launchpad.net/juniperopenstack/+bug/1333404

BgpXmppChannel::DeferQFlush does not work correctly if only the
VRF name is provided and the VRF in question is not the VRF for
the first entry in the defer_q_. This could cause a memory leak
or incorrect routes on the control node.