drlg_l1: add BUGFIX for DRLG_L5

[mewmew]

When generating dlvl=2 dungeons containing a
staircase down to the Poisoned Water Supply,
a check is missing to ensure that doneflag
is not set to FALSE by the first call to
DRLG_PlaceMiniSet(PWATERIN, ...).

If that was the case, then a second call to
DRLG_PlaceMiniSet may be made, to place
L5STAIRSUP even though doneflag may have been
set to FALSE.

For the correct logic, cross reference against
the drlg_l2 code for placing staircases using
DRLG_L2PlaceMiniSet.

For a PoC triggering the incorrect behaviour,
see dlvl=2, seed=0x0000007B with
quest=Poisoned Water Supply active.

[galaxyhaxz]

So technically it would be possible to get a map that can't spawn the poisoned water entrance but fits the stairs meaning the quest is active but there's no way to reach it....

I guess the townspeople will have to drink something else xD