Add NULL handling for the gpio_num pointer.

Fixes #1050.

src/relay_chn.c

@@ -298,6 +298,8 @@ static esp_err_t relay_chn_create_event_loop()
 
 esp_err_t relay_chn_create(const gpio_num_t* gpio_map, uint8_t gpio_count)
 {
+    ESP_RETURN_ON_FALSE(gpio_map, ESP_ERR_INVALID_ARG, TAG, "gpio_map cannot be NULL");
+
     // Check if the device's GPIOs are enough for the number of channels
     if (RELAY_CHN_COUNT > (GPIO_PIN_COUNT / 2)) {
         ESP_LOGE(TAG, "Not enough GPIOs for the number of channels!");