Blender Git Loki

UI: Fix crash using drag-toggle over window bounds with button callback

Steps to reproduce:
* Go to modifier context in properties editor
* Add modifier, collapse it
* Press down LMB over collapse button of modifier, hold it
* Drag over pin-icon in properties editor (to keep fixed data-block displayed)
* Drag outside of window bounds (should crash)

Also could've solved by getting space data from callback arguments instead of context, but this fix is much nicer (though not totally un-risky).