esp32/network_ppp: Block after deleting task.

When calling ppp.active(False) we could get a crash due to immediately
returning after asking FreeRTOS to delete the current task.

This commit adds a simple blocking loop, the same as used in all other
places where we call vTaskDelete(NULL).

Signed-off-by: Daniël van de Giessen <daniel@dvdgiessen.nl>

ports/esp32/network_ppp.c

@@ -114,6 +114,8 @@ static void pppos_client_task(void *self_in) {
 
     self->client_task_handle = NULL;
     vTaskDelete(NULL);
+    for (;;) {
+    }
 }
 
 STATIC mp_obj_t ppp_active(size_t n_args, const mp_obj_t *args) {