To summarize what we know:
Reproduction:
* Start tutorial
* Talk to TuTom, select "No thanks, I still remember how to walk..." node.
* Pick up the 3 items from the first section, proceed, loot the chest and pick
up the items therein.
* Talk to TuTom. Repeatedly select the first node and click through the dialog;
eventually the trade screen will appear. Trade as instructed.
* Proceed to the melee training. Talk to TuTom, select the first node repeatedly
as before until the dialog ends.
* Kill the 123 and bring its drop as instructed; proceed to the next bot and repeat.
* Select the "I'm ready to move on." node; the game will freeze (or crash with
ASAN).

The actual freeze occurs when the game calls end_quest(); in there, it fails in
complete_quest(); in that function, defined in mission.c, it fails to execute
this line:
run_lua(LUA_DIALOG, Me.AllMissions[MissNum].completion_lua_code);
In turn, the completion code for the melee mission only involved
display_big_message(). This is as far as testing got.