#2036 Crash when switching collections in slice collection editor

[TeeEmCee]

sotrain515 reported repeatedly seeing the slice collection editor crash while switching slice collections.
https://www.slimesalad.com/forum/viewtopic.php?t=7626
Using etheldreme.

1.) Opened custom.exe
2.) Loaded the game I'm working on (file named "a_tactics.rpg")
3.) Navigated right to Slice Collection Editor.
4.) In the Collection Editor moved down to the "<- Slice Collection ->" picker part.
5.) Typed "2", "6" as I wanted to navigate right to collection 26.
6.) Custom crashed right out to desktop with no error message.

[TeeEmCee]

Fix in r11141 with help from SwordPlay. Was due to a stale pointer for the slice under the mouse, but having the mouse over a slice was not enough to trigger it, it was still rare.
Typically it manifested as a freeze instead of a crash, due to passing enormous numbers to drawants.