Code in lib/fts.c looks like this:

-----
602 next:   tmp = p;
603         if ((p = p->fts_link) != NULL) {
604                 free(tmp);
605 
606                 /*
607                  * If reached the top, return to the original directory (or
608                  * the root of the tree), and load the paths for the next root.
609                 */
610                 if (p->fts_level == FTS_ROOTLEVEL) {
611                         if (FCHDIR(sp, sp->fts_rfd)) {
612                                 SET(FTS_STOP);
613                                 return (NULL);
614                         }
615                         fts_load(sp, p);
616                         if (p->fts_info == FTS_D)
617                                 ENTER_DIR (sp, p, "8");
618                         return (sp->fts_cur = p);
619                 }
----

Basically we free() set 'p = p->fts_link' on line 603, and then free the
old 'p' on line 604, but then if we fail to fchdir() on line 611,  we do
not update 'sp->fts_cur' ...  Thus update 'sp->fts_cur' before we return
NULL on line 613.  (bug #87490)

--- coreutils-5.2.1/lib/fts.c
+++ coreutils-5.2.1/lib/fts.c
@@ -610,6 +610,7 @@
 		if (p->fts_level == FTS_ROOTLEVEL) {
 			if (FCHDIR(sp, sp->fts_rfd)) {
 				SET(FTS_STOP);
+				sp->fts_cur = p;
 				return (NULL);
 			}
 			fts_load(sp, p);