Reject intervals that are out of order

If an interval if out of order, it ends up emitting 0000-00-00 00:00:00
as the "next" time, which is undesirable. This patch updates Parse to
simply reject such intervals.

Since the rejection reason might not be super obvious (the range 6-7 is
actually translated to 6-0, which makes it invalid), let's also print
the original vs. normalized form for clarity.

See: https://github.com/aptible/supercronic/issues/63

cronexpr_parse.go

@@ -438,6 +438,9 @@ func genericFieldParse(s string, desc fieldDescriptor) ([]*cronDirective, error)
 			directive.first = desc.atoi(snormal[pairs[2]:pairs[3]])
 			directive.last = desc.atoi(snormal[pairs[4]:pairs[5]])
 			directive.step = 1
+			if directive.last < directive.first {
+				return nil, fmt.Errorf("invalid interval %s (normalized to %d-%d)", snormal, directive.first, directive.last)
+			}
 			directives = append(directives, &directive)
 			continue
 		}

cronexpr_test.go

@@ -305,6 +305,19 @@ func TestInterval_Interval60Issue(t *testing.T) {
 	}
 }
 
+// Issue: https://github.com/aptible/supercronic/issues/63
+func TestRange_OutOfOrder(t *testing.T) {
+	_, err := Parse("45 4 *  *  6-7")
+	if err == nil {
+		t.Errorf("parsing with range 6-7 should return err")
+	}
+
+	_, err = Parse("45 4 *  *  6-5")
+	if err == nil {
+		t.Errorf("parsing with range 6-5 should return err")
+	}
+}
+
 /******************************************************************************/
 
 func TestTooManyFields(t *testing.T) {