serial: protect access to serial rx buffer

Add test to avoid access to rx buffer when this buffer is empty.
In this case directly call getc() function to avoid issue when tstc()
is not called.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/serial/serial-uclass.c

@@ -228,6 +228,9 @@ static int _serial_getc(struct udevice *dev)
 	struct serial_dev_priv *upriv = dev_get_uclass_priv(dev);
 	char val;
 
+	if (upriv->rd_ptr == upriv->wr_ptr)
+		return __serial_getc(dev);
+
 	val = upriv->buf[upriv->rd_ptr++];
 	upriv->rd_ptr %= CONFIG_SERIAL_RX_BUFFER_SIZE;