bhyve: Avoid passing a possible garbage pointer to free().

All of the error paths in pci_vtcon_sock_add free the sock pointer.
However, sock is not initialized until part way through the function.
An early error would pass stack garbage to free().

Reviewed by:	corvink, markj
Differential Revision:	https://reviews.freebsd.org/D37491

usr.sbin/bhyve/pci_virtio_console.c

@@ -273,7 +273,7 @@ static int
 pci_vtcon_sock_add(struct pci_vtcon_softc *sc, const char *port_name,
     const nvlist_t *nvl)
 {
-	struct pci_vtcon_sock *sock;
+	struct pci_vtcon_sock *sock = NULL;
 	struct sockaddr_un sun;
 	const char *name, *path;
 	char *cp, *pathcopy;