bhyve: Don't free an invalid pointer.

The netmap-specific data stored at be->opaque is freed by the caller
on error as part of freeing be.

Reviewed by:	markj
Reported by:	GCC -Wfree-nonheap-object
Differential Revision:	https://reviews.freebsd.org/D36828

usr.sbin/bhyve/net_backends.c

@@ -695,7 +695,6 @@ netmap_init(struct net_backend *be, const char *devname,
 	if (priv->nmd == NULL) {
 		WPRINTF(("Unable to nm_open(): interface '%s', errno (%s)",
 			devname, strerror(errno)));
-		free(priv);
 		return (-1);
 	}