fts: Fix a potential memory leak in error case

Dan Krejsa reports a potential memory leak in an fts_build error case,
detected by Coverity.  (It doesn't seem to show up in Coverity Scan, so I
don't have a CID to point to.)

I don't know whether it is actually possible to arrive in this case with a
non-empty 'head' list.  The cost is low, though.  One additional branch in a
terminal error case isn't the end of the world.

PR:		217125
Submitted by:	Dan Krejsa <dan.krejsa at gmail.com>

lib/libc/gen/fts.c

@@ -850,6 +850,7 @@ mem1:				saved_errno = errno;
 	    (cur->fts_level == FTS_ROOTLEVEL ?
 	    FCHDIR(sp, sp->fts_rfd) :
 	    fts_safe_changedir(sp, cur->fts_parent, -1, ".."))) {
+		fts_lfree(head);
 		cur->fts_info = FTS_ERR;
 		SET(FTS_STOP);
 		return (NULL);