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

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.

Descent3/BOA.cpp

@@ -329,6 +329,8 @@ int BOA_DetermineStartRoomPortal(int start_room, vector *start_pos, int end_room
                                  bool f_making_robot_path_invalid_list, int *blocked_portal) {
   int i;
 
+  if (start_room == -1 || end_room == -1)
+    return -1;
   if (start_room > Highest_room_index && end_room > Highest_room_index)
     return -1;
 
@@ -566,12 +568,12 @@ int BOA_GetNextRoom(int start_room, int end_room) {
   int e_index = end_room;
 
   if (start_room == -1 || end_room == -1) {
-    return false;
+    return BOA_NO_PATH;
   }
 
   if ((!ROOMNUM_OUTSIDE(s_index)) && s_index <= Highest_room_index) {
     if (!Rooms[s_index].used) {
-      return false;
+      return BOA_NO_PATH;
     }
   } else if (ROOMNUM_OUTSIDE(s_index)) {
     s_index = TERRAIN_REGION(start_room) + Highest_room_index + 1;
@@ -581,7 +583,7 @@ int BOA_GetNextRoom(int start_room, int end_room) {
 
   if ((!ROOMNUM_OUTSIDE(e_index)) && e_index <= Highest_room_index) {
     if (!Rooms[e_index].used) {
-      return false;
+      return BOA_NO_PATH;
     }
   } else if (ROOMNUM_OUTSIDE(e_index)) {
     e_index = TERRAIN_REGION(end_room) + Highest_room_index + 1;

Descent3/aipath.cpp

@@ -60,6 +60,8 @@ static void AIUpdatePathInfo(q_item **node_list, int start, int end) {
 
 // Ok to use Highest_room_index offset stuff
 bool AIFindAltPath(object *obj, int i, int j, float *dist) {
+  if (i == -1 || j == -1)
+    return false;
   i = BOA_INDEX(i);
   j = BOA_INDEX(j);
 

Descent3/bnode.cpp

@@ -210,6 +210,8 @@ static void BNode_UpdatePathInfo(pq_item **node_list, int start, int end) {
 
 // Ok to use Highest_room_index offset stuff
 bool BNode_FindPath(int start_room, int i, int j, float rad) {
+  if (start_room == -1)
+    return false;
   bpq PQPath;
   int counter;
   pq_item *start_node = new pq_item(i, -1, 0.0f);
@@ -311,6 +313,8 @@ int BNode_FindDirLocalVisibleBNode(int roomnum, vector *pos, vector *fvec, float
   }
 
   bn_list *bnlist = BNode_GetBNListPtr(roomnum);
+  if (bnlist == nullptr)
+    return -1;
 
 retry:
 
@@ -402,6 +406,7 @@ int BNode_FindClosestLocalVisibleBNode(int roomnum, vector *pos, float rad) {
   }
 
   bn_list *bnlist = BNode_GetBNListPtr(roomnum);
+  ASSERT(bnlist);
 
 retry: