Add extra checking into A_SpawnSound function

before calling the A_SpawnFly function.
Batman TC calls A_SpawnSound during the death sequence of BANE in map 30.
The old version on A_SpawnFly must've fluked a safe exit, the new one goes badly wrong and the game fails to exit.

Source/p_enemy.c

@@ -2569,8 +2569,13 @@ void A_SpawnFly (mobj_t* mo, pspdef_t* psp)
 // travelling cube sound
 void A_SpawnSound (mobj_t* mo, pspdef_t* psp)
 {
-    S_StartSound (mo,sfx_boscub);
-    A_SpawnFly(mo,psp);
+  S_StartSound (mo,sfx_boscub);
+
+  /* Note: Batman TC calls this function when BANE dies at the end */
+  /* of map 30. If we let it call A_SpawnFly then it's bad news! */
+
+  if (mo->type == MT_SPAWNSHOT)
+    A_SpawnFly (mo,psp);
 }