Handle invalid ranges correctly

This fixes #114

Author: kiorky

src/croniter/croniter.py

@@ -511,7 +511,11 @@ def expand(cls, expr_format):
                             "[{0}] is not acceptable".format(expr_format))
 
                     low, high, step = map(int, [low, high, step])
-                    rng = range(low, high + 1, step)
+                    try:
+                        rng = range(low, high + 1, step)
+                    except ValueError as exc:
+                        raise CroniterBadCronError(
+                            'invalid range: {0}'.format(exc))
                     e_list += (["{0}#{1}".format(item, nth) for item in rng]
                         if i == 4 and nth else rng)
                 else:

src/croniter/tests/test_croniter.py

@@ -832,6 +832,9 @@ def test_milliseconds(self):
         ts = "{0}".format(datetime.utcfromtimestamp(c.get_prev()))
         self.assertEqual(ts, '2018-01-01 10:00:00')
 
+    def test_invalid_zerorepeat(self):
+        self.assertFalse(croniter.is_valid('*/0 * * * *'))
+
 
 if __name__ == '__main__':
     unittest.main()