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>

Author: Andy King

drivers/misc/vmw_vmci/vmci_queue_pair.c

@@ -3355,11 +3355,11 @@ ssize_t vmci_qpair_dequev(struct vmci_qp *qpair,
 {
 	ssize_t result;
 
-	qp_lock(qpair);
-
 	if (!qpair || !iov)
 		return VMCI_ERROR_INVALID_ARGS;
 
+	qp_lock(qpair);
+
 	do {
 		result = qp_dequeue_locked(qpair->produce_q,
 					   qpair->consume_q,