serial: nrfx_uarte: check size before writing to buffer

NRFX UARTE would write to user supplied buffer on IRQ without checking
whether or not the supplied buffer had available space left to write
one char

Signed-off-by: Arne Edholm <arne.edholm@assaabloy.com>

drivers/serial/uart_nrfx_uarte.c

@@ -1452,7 +1452,7 @@ static int uarte_nrfx_fifo_read(const struct device *dev,
 	NRF_UARTE_Type *uarte = get_uarte_instance(dev);
 	const struct uarte_nrfx_data *data = get_dev_data(dev);
 
-	if (nrf_uarte_event_check(uarte, NRF_UARTE_EVENT_ENDRX)) {
+	if (size > 0 && nrf_uarte_event_check(uarte, NRF_UARTE_EVENT_ENDRX)) {
 		/* Clear the interrupt */
 		nrf_uarte_event_clear(uarte, NRF_UARTE_EVENT_ENDRX);