fix: add null checks before removing parent refs

This is to prevent the disable method to fail if there is anything on
the require cache object that does not have a parent element or
children on that parent object.

This fixes a problem introduced on PR
#57

mockery.js

@@ -341,7 +341,7 @@ function removeParentReferences() {
         if (k.indexOf('\.node') === -1) {
             // don't touch native modules, because they're special
             var mod = m._cache[k];
-            var idx = mod.parent.children.indexOf(mod);
+            var idx = mod.parent && mod.parent.children && mod.parent.children.indexOf(mod);
             if (idx > -1) {
                 mod.parent.children.splice(idx, 1);
             }