Fix DISPLAY

SMILEBASIC/builtinfunctions.d

@@ -414,6 +414,8 @@ class BuiltinFunction
     }
     static void DISPLAY(PetitComputer p, int display)
     {
+        if (p.currentDisplay.count <= display || display < 0)
+            throw new OutOfRange("DISPLAY", 1);
         p.display(display);
     }
     static int DISPLAY(PetitComputer p)

SMILEBASIC/petitcomputer.d

@@ -174,6 +174,10 @@ struct Display
     SDL_Rect[] rect;
     Size windowSize;
     int yoffset;
+    int count()
+    {
+        return cast(int)rect.length;
+    }
 }
 class RingBuffer(T)
 {