chore($parse): micro-optimization for ensureSafeObject function

This version matches the "alternate 2.2" version here: http://jsperf.com/ensuresafeobject/2

alternate 2.3 is a bit faster and simpler, but would break backwards compatibility.

Closes #5246
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!