fix panic in field generation

There is an error in the regex detection of whether an expression is
valid or not. This makes it dangerous for downstream functions to access
the fields within an expression.

This adds a quick length check as a backup check for the potentially
leaky regex.

Closes #1

avail.go

@@ -47,6 +47,10 @@ func New(expression string) (Avail, error) {
 	}
 
 	terms := strings.Split(expression, " ")
+	// we need this extra check to make sure there are the proper amount of fields because I am bad at regex
+	if len(terms) != 6 {
+		return Avail{}, fmt.Errorf("could not parse cron expression: %s; must have 6 terms", expression)
+	}
 
 	minutes, err := newField(minute, terms[0], 0, 59)
 	if err != nil {

avail_test.go

@@ -46,6 +46,9 @@ func TestUnparseable(t *testing.T) {
 		"too few arguments": {
 			expression: "* * * *",
 		},
+		"too few arguments w/ value": {
+			expression: "* 14 * * *",
+		},
 		"out of bounds single value": {
 			expression: "* * * * 22222 *",
 		},