d3: fix game crash in Retribution level 12's big matcen room

[jengelh]

When activating switches in the room with 6 matcens, this happens after a short while:

Assertion failure at TERRAIN_REGION (Descent3/terrain.h:218), triggered 1 time:
  'x != -1 && "invalid/unset room number (-1)!"'

f0  TERRAIN_REGION (x=-1) at Descent3/terrain.h:222
f1  AIPathAllocPath (obj=0x3d304b0 <Objects+150000>, ai_info=0x520000444080, goal_ptr=0x520000444250, start_room=0x3d304cc <Objects+150028>, start_pos=0x3d304d0 <Objects+150032>, end_room=0x7fb115dbe830, end_pos=0x52000044427c, rad=0, flags=0, handle=28822, ignore_obj=-1) at Descent3/aipath.cpp:1066
f2  GoalDoFrame (obj=0x3d304b0 <Objects+150000>) at Descent3/AIGoal.cpp:823
f3  AIDoFrame (obj=0x3d304b0 <Objects+150000>) at Descent3/AImain.cpp:6214
f4  ObjDoFrame (obj=0x3d304b0 <Objects+150000>) at Descent3/object.cpp:2674
f5  ObjDoFrameAll () at Descent3/object.cpp:2988
f6  GameFrame () at Descent3/GameLoop.cpp:2981
f7  GameSequencer () at Descent3/gamesequence.cpp:1221
f8  PlayGame () at Descent3/game.cpp:834
f9  MainLoop () at Descent3/descent.cpp:550
f10 Descent3 () at Descent3/descent.cpp:508
f11 oeD3LnxApp::run (this=0x7fb115a0db50) at Descent3/sdlmain.cpp:151

(gdb) up
    start_room=0x3d304cc <Objects+150028>, start_pos=0x3d304d0 <Objects+150032>, end_room=0x7fb115dbe830, end_pos=0x52000044427c, rad=0, flags=0,
    handle=28822, ignore_obj=-1) at Descent3/aipath.cpp:1066
1066          } else if (BOA_Array[BOA_INDEX(*start_room)][BOA_INDEX(*end_room)] & BOAF_TOO_SMALL_FOR_ROBOT) {
(gdb) p *start_room
$2 = 171
(gdb) p *end_room
$3 = -1

The return type of BOA_GetNextPath is int; but inside the function, false is returned, which does not fit the scheme. Judging from callsites, BOA_NO_PATH is expected instead.

In some functions related to AI pathfinding, add handling or assertions for invalid room numbers.

Pull Request Type

• [x] Runtime changes
  • [x] Other changes

Checklist

• [x] I have tested my changes locally and verified that they work as intended.
• [x] I have reviewed the changes to ensure they do not introduce any unnecessary complexity or duplicate code.
• [x] I understand that by submitting this pull request, I am agreeing to license my contributions under the project's license.