Fix out-of-bounds access

src/picoled.cpp

@@ -16,6 +16,8 @@ oled::~oled()
 
 pixel& oled::operator()(int x, int y)
 {
+  static pixel dummy;
+  if (x < 0 || x >= width || y < 0 || y >= height) { return dummy; }
   return pixels[x + y * width];
 }