Fix crypto support detection doesn't work with jest

Some testing framework define a global `window` object but not necessarily `window.crypto`.
That's the case of Facebook's Jest. ulid should use Node's `crypto` module even when `window` is defined
but `window.crypto` or `window.msCrypto` are not defined.

Author: giuseppeg

index.js

@@ -81,18 +81,18 @@ function factory(prng) {
 
 /* istanbul ignore next */
 function _prng(root) {
+  var crypto = root && (root.crypto || root.msCrypto)
 
-  if (root) {
+  if (crypto) {
     try {
-      var crypto = root.crypto || root.msCrypto
       return function() {
         return crypto.getRandomValues(new Uint16Array(1))[0] / 0xFFFF
       }
     }
     catch (e) {}
   } else {
     try {
-      var crypto = require("crypto")
+      crypto = require("crypto")
       return function() {
         return crypto.randomBytes(2).readUInt16LE() / 0xFFFF
       }