Took me some time to figure out what was going on here... Was again that delayed button
callback stuff (`ui_apply_but_funcs_after()`), first calling button op, and then
its callback func.
Issue was that 'open file' op (through call to `WM_file_read()`) would clear
the splash screen (as more or less the entire 'dynamic' UI), but callback func of that splash
(`wm_block_splash_refreshmenu()`) would still try to access that freed menu's region.
So, root of the issue seems to be that setting context's wm/win/etc. would not clear
context's menu pointer (while clearing all other 'sub' pointers). I could not find
nor imagine any case where this behavior could be desired, so simply added nullification
of that pointer when setting context's wm/win/etc.
Note that crash was due to read-after-free, infuriating debug builds with asan,
but seems like release builds never actually crashed on it.
935e241fa6