FreeBSD: Fix potential boot panic with bad label

vdev_geom_read_pool_label() can leave NULL in configs.  Check for it
and skip consistently when generating rootconf.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes openzfs#14291

module/os/freebsd/zfs/spa_os.c

@@ -94,6 +94,8 @@ spa_generate_rootconf(const char *name)
 	for (i = 0; i < count; i++) {
 		uint64_t txg;
 
+		if (configs[i] == NULL)
+			continue;
 		txg = fnvlist_lookup_uint64(configs[i], ZPOOL_CONFIG_POOL_TXG);
 		if (txg > best_txg) {
 			best_txg = txg;