tr: set2 must be set in some cases

echo abc | tr 'a' '' would crash because of:

	m--;
	r = set2[m].start + (off1 - off2) / set2[m].quant;

if set2ranges > 0 it's fine.

tr.c

@@ -196,6 +196,8 @@ main(int argc, char *argv[])
 	set1ranges = makeset(argv[0], &set1);
 	if (argc == 2)
 		set2ranges = makeset(argv[1], &set2);
+	if (!dflag && !set2ranges)
+		eprintf("set2 must be non-empty\n");
 read:
 	if (!readrune("<stdin>", stdin, &r))
 		return 0;