Fix crash in hlsSystem::ComputePlayInfo(), DescentDevelopers#141

sound_seg was -1, which crashes in BOA_INDEX(sound_seg) on 64bit
platforms (by pure luck it doesn't seem to crash on 32bit platforms)

It makes sense to catch this problem much earlier in Play3dSound(),
to avoid executing all the superfluous

sndlib/hlsoundlib.cpp

@@ -922,6 +922,16 @@ bool hlsSystem::ComputePlayInfo(int sound_obj_index, vector *virtual_pos, vector
     sound_seg = m_sound_objects[sound_obj_index].m_link_info.pos_info.segnum;
   }
 
+  // sound_seg == -1 causes crashes when BOA_INDEX() calls TERRAIN_REGION()
+  // with that value (though by pure luck on 32bit platforms the overflow
+  // and truncation will likely use an address that doesn't crash,
+  // but is still invalid). At least one case that could cause this was fixed,
+  // if there are others, the ASSERT should tell us about it
+  // (and if assertions are disabled, return false to handle this gracefully)
+  ASSERT(sound_seg != -1);
+  if (sound_seg == -1)
+    return false;
+
   sound_seg = BOA_INDEX(sound_seg);
   ear_seg = BOA_INDEX(Viewer_object->roomnum);
   if (!BOA_IsSoundAudible(sound_seg, ear_seg))
@@ -1079,6 +1089,11 @@ int hlsSystem::Play3dSound(int sound_index, pos_state *cur_pos, object *cur_obj,
     return -1;
   if (sound_index >= MAX_SOUNDS || Sounds[sound_index].used == 0)
     return -1;
+  // if the position doesn't belong to any valid room or cell,
+  // all this would fail anyway (in Emulate3dSound() -> ComputePlayInfo()),
+  // so might as well give up now
+  if (cur_pos->roomnum == -1)
+    return -1;
   // initialize sound.
   Sound_system.CheckAndForceSoundDataAlloc(sound_index);
   int sample_offset = offset * 22050.0f;