bi-directional: avoid leaking already-allocated xprt handle
bi-directional: avoid leaking already-allocated xprt handle

The case is logically possible, so prevent overwriting the just-ref'd
xprt handle, and return it.

The new done_xprt label is used in a following commit.

Signed-off-by: Matt Benjamin <b5ea062884a8f3dc969ff57d1008615447c56296@redhat.com>

sswen/ntirpc,sswen/ntirpc