Fixed: Possible invalid processing for Crash states that immediately destroy the actor

- Setting an actor's Crash state has the potential to destroy the actor if
  the Crash state has one or more 0-tic states that end with Stop. This
  was not taken into account when the object's Z velocity was 0, but it
  was under the floor anyway.

src/p_mobj.cpp

@@ -3799,6 +3799,8 @@ void AActor::Tick ()
 		else if (Z() <= floorz)
 		{
 			Crash();
+			if (ObjectFlags & OF_EuthanizeMe)
+				return;		// actor was destroyed
 		}
 
 		CheckPortalTransition(true);