Fix incrementing invalid property in _hive()

I've run into a really crazy data corruption issue that looks to be caused by running dpm() on an object. I was dpm'ing $data inside of some token_replace() code, and it was causing the uid of the user object to be set to 1. I think it might actually be a PHP bug, but I can't reproduce it outside of my site code, though it does happen on both Ubuntu's PHP 5.5.9 and a PPA for 5.5.16.

Inside of krumo, there's a _hive() method that helps to handle recursive object references. It attempts to increment a variable that hasn't been set, while silencing the notice at the same time. The actual line that was modifying the uid is ? @($bee->$_recursion_marker++). Also, the actual recursion_marker value never gets set, though in a test script incrementing a missing property on an object is setting it to 1.

Rather than continue debugging to see why this is happening, I've cleaned up the code to be a little more defensive so it doesn't need to silence the notice in the first place. This patch applies against both 8.x and 7.x.