btrfs.c: cleanup use of tcp->auxstr

* btrfs.c (btrfs_ioctl): Reset tcp->auxstr only if it is set.
diff --git a/btrfs.c b/btrfs.c
index 1eed651..5a4c76f 100644
--- a/btrfs.c
+++ b/btrfs.c
@@ -860,11 +860,11 @@
 
 		tprints("{");
 		if (tcp->auxstr) {
+			tcp->auxstr = NULL;
 			tprints("treeid=");
 			btrfs_print_objectid(args.treeid);
 			tprints(", ");
 		}
-		tcp->auxstr = NULL;
 
 		tprints("name=");
 		print_quoted_string(args.name, sizeof(args.name),