Fix tus_file_shutdown

we removed an item from the tree while iterating it.

Spotted by flexelint with stricter rules, see follow up commits.

See tree(3bsd)

src/tus_file.c

@@ -679,11 +679,14 @@ tus_file_exp(struct tus_file_core **fcorep)
 void
 tus_file_shutdown(struct VPFX(tus_server) *srv)
 {
-	struct tus_file_core *fcore;
+	struct tus_file_core *fcore, *next;
 	struct tus_files *files = tus_server_files(srv);
 
 	tus_server_lock(srv);
-	VSPLAY_FOREACH(fcore, tus_files, files) {
+	for (fcore = VSPLAY_MIN(tus_files, files);
+	     fcore != NULL;
+	     fcore = next) {
+		next = VSPLAY_NEXT(tus_files, files, fcore);
 		REPLACE(fcore->filename, NULL); // prevent unlink
 		Lck_Lock(&fcore->mtx);
 		tus_file_del_shutdown(&fcore);