Avoid negative array indices: an empty string can also be used

to specify a default value.

usr.bin/jot/jot.1

@@ -106,7 +106,8 @@ and the step size or, for random data, the seed.
 While at least one of them must appear,
 any of the other three may be omitted, and
 will be considered as such if given as
-.Fl "" .
+.Fl ""
+or as an empty string.
 Any three of these arguments determines the fourth.
 If four are specified and the given and computed values of
 .Ar reps

usr.bin/jot/jot.c

@@ -72,7 +72,7 @@ __FBSDID("$FreeBSD$");
 #define HAVE_BEGIN	4
 #define HAVE_REPS	8
 
-#define	is_default(s)	(strcmp((s), "-") == 0)
+#define	is_default(s)	(*(s) == 0 || strcmp((s), "-") == 0)
 
 double	begin;
 double	ender;