Crash when removing inactive touches

[toddreed]

This was reported in a comment by @gatamar for #16, but I think it’s a separate bug.

The issue is with line 85:

https://github.com/toddreed/Touchpose/blob/dd2b7293ec5cf052dc16594cc1f8ecd56a1e328d/Pod/Source/TRTouchposeApplication.m#L80-L86

which would attempt to retain an object that may no longer exist.