convertSteps function miss cases of a string as a divider

[ZimGil]

I ran into a case where I passed an undefined value as part of my expression as */undefined * * * * and cron.validate() return true and find that it behave the same as a normal * * * * *

If I understand correctly, in this case the convertSteps function should receive 1,2,3,4,5,...,59/undefined which is not getting picked up by the ^(.+)\/(\d+)$ regex.

node-cron/src/convert-expression/step-values-conversion.js

Lines 4 to 13 in 2a39703

	function convertSteps(expressions){
	var stepValuePattern = /^(.+)\/(\d+)$/;
	for(var i = 0; i < expressions.length; i++){
	var match = stepValuePattern.exec(expressions[i]);
	var isStepValue = match !== null && match.length > 0;
	if(isStepValue){
	var values = match[1].split(',');
	var setpValues = [];
	var divider = parseInt(match[2], 10);
	for(var j = 0; j <= values.length; j++){

Later it gets to the normalizeIntegers function and: parseInt('59/undefined') => 59

Proposed Solution:
We can change the regex to ^(.+)\/(\w+)$ and check as early as possible if isNaN(divider) and throw an error in other cases.