npc.php: fix invalid ship ID with Illusion Generator
npc.php: fix invalid ship ID with Illusion Generator

When setting the Illusion Generator ship ID for NPCs, we were selecting
from among the array _keys_ of an array of valid ship IDs, instead of
the array _values_. This was causing the NPCs to sometimes set the IG
ship ID to 0, which threw an exception when trying to get the ship type
of that ID.

hemberger/smr,smrealms/smr,hemberger/smr,hemberger/smr,smrealms/smr,hemberger/smr,smrealms/smr