app: minor, make line select more secure

application.c

@@ -1229,7 +1229,7 @@ void RGBMATRIX_ISR(void)
 {
 	if (dma_get_interrupt_flag(RGBMATRIX_DMA, RGBMATRIX_STREAM, DMA_TCIF)) {
 		dma_clear_interrupt_flags(RGBMATRIX_DMA, RGBMATRIX_STREAM, DMA_TCIF);
-		GPIOB_ODR = (GPIOB_ODR & 0xfff0) + rgbmatrix_line; // select line (line on lower and upper half are updated at once)
+		GPIOB_ODR = (GPIOB_ODR & 0xfff0) | (rgbmatrix_line & 0xf); // select line (line on lower and upper half are updated at once)
 		rgbmatrix_line = (rgbmatrix_line + 1) % (RGBMATRIX_HEIGHT / 2); // go to next line (two lines are updated at once)
 	}
 }