Rare Segfault Bugfix

Preconditions
 - "Button 3 Mapping" set to "Select Region".
 - "Hand Tool" selected.

Steps to reproduce:
 1. Right click (button 3) and hold, dragging the select region off the canvas.
 2. Without re-entering the canvas, "File"->"Annotate PDF", and select a
PDF to be the new background.
 3. Re-enter the canvas with the mouse pointer
 4. Segfault