Avoid hang in stdio_set_chars_available_callback

The function is setting the callback after enabing interrupts which
can cause a hang if a receive character is already pending.
Smilarly we also have to clear the callback pointer only after the
interrupt is disabled.

Fixes raspberrypi#2009

src/rp2_common/pico_stdio_uart/stdio_uart.c

@@ -172,16 +172,18 @@ static void on_uart_rx(void) {
 static void stdio_uart_set_chars_available_callback(void (*fn)(void*), void *param) {
     uint irq_num = UART_IRQ_NUM(uart_instance);
     if (fn && !chars_available_callback) {
+        chars_available_callback = fn;
+        chars_available_param = param;
         irq_set_exclusive_handler(irq_num, on_uart_rx);
         irq_set_enabled(irq_num, true);
         uart_set_irqs_enabled(uart_instance, true, false);
     } else if (!fn && chars_available_callback) {
         uart_set_irqs_enabled(uart_instance, false, false);
         irq_set_enabled(irq_num, false);
         irq_remove_handler(irq_num, on_uart_rx);
+        chars_available_callback = fn;
+        chars_available_param = param;
     }
-    chars_available_callback = fn;
-    chars_available_param = param;
 }
 #endif