code: only call Date.parse(val) once

If it's going to create a problem preventing the debug call then it'll
create the problem even when it's the last arg to debug. So, might as
well do it once, first, and then use that result in the debug call.

Credit: @elidoran
Reviewed-By: @othiym23
PR-URL: #76

lib/nopt.js

@@ -146,8 +146,8 @@ function validateNumber (data, k, val) {
 }
 
 function validateDate (data, k, val) {
-  debug("validate Date %j %j %j", k, val, Date.parse(val))
   var s = Date.parse(val)
+  debug("validate Date %j %j %j", k, val, s)
   if (isNaN(s)) return false
   data[k] = new Date(val)
 }