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.

Signed-off-by: Ryan Moeller <ryan@iXsystems.com>

module/os/freebsd/zfs/spa_os.c

@@ -94,12 +94,20 @@ 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;
 			best_cfg = configs[i];
 		}
 	}
+	if (best_cfg == NULL) {
+		for (i = 0; i < count; i++)
+			fnvlist_free(configs[i]);
+		kmem_free(configs, count * sizeof (void *));
+		return (NULL);
+	}
 
 	nchildren = 1;
 	nvlist_lookup_uint64(best_cfg, ZPOOL_CONFIG_VDEV_CHILDREN, &nchildren);