VMCI: Fix deref before NULL-check of queuepair ptr

Check for a valid queuepair ptr before trying to lock the queuepair (which will
deref it).

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andy King <acking@vmware.com>
Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>