EngineCrash [master f83afe1] at RenderUnitDestroyed on synthetic test

Found a case where the engine crashes, probably not very worrying since it just happens on a synthetic test and likely the situation won't happen in a real game.

crash at: `rts/Lua/LuaHandle.cpp:1925 CLuaHandle::RenderUnitDestroyed(CUnit const*)`

Seems to be because of clearing the map just after the unit selfd, likely between when the unit is "destroyed" and the callin runs. I guess the unit is expected to still be there but the test map clearing already deleted it itself.

### Reproducing

- Take [test_crash.lua](https://github.com/saurtron/Beyond-All-Reason/blob/test-crash/luaui/Widgets/Tests/crash/test_crash.lua) and place under BAR.sdd/luaui/Widgets/Tests/crash/
- Start skirmish in neverend mode
- Call `/runtests test_crash`

Should crash during the first run, although the test runs the procedure 5 times just in case.

Start at rts/Lua/LuaHandle.cpp:1925 in CLuaHandle::RenderUnitDestroyed and reproduce the crash with test_crash.lua under BAR.sdd/luaui/Widgets/Tests/crash/. Inspect the timing between unit self-destruction, test-map clearing, and the callback. Done means the synthetic test can run repeatedly without the engine crashing.

Written by the indexing model from the issue text.