Make mines simply disappear when they time out rather than exploding.

The explosions of mines off in the distance are extremely annoying in
the Mars mission, in particular. There's no real point in them
doing a timed off-screen explosion, anyway; if they just disappear,
the player won't notice.

src/game.cpp

@@ -481,7 +481,9 @@ static void game_doCollectables()
 		}
 		else
 		{
-			if (collectable->type == P_MINE)
+			if ((collectable->type == P_MINE) && (collectable->x >= 0) &&
+					(collectable->x <= screen->w) && (collectable->y >= 0) &&
+					(collectable->y <= screen->h))
 				collectable_explode(collectable);
 			prevCollectable->next = collectable->next;
 			delete collectable;