Make sure that WiFi is started before trying to erase configuration.

Connected to: https://github.com/espressif/arduino-esp32/issues/400

libraries/WiFi/src/WiFiSTA.cpp

@@ -260,6 +260,7 @@ bool WiFiSTAClass::disconnect(bool wifioff)
     *conf.sta.password = 0;
 
     WiFi.getMode();
+    esp_wifi_start();
     esp_wifi_set_config(WIFI_IF_STA, &conf);
     ret = esp_wifi_set_config(WIFI_IF_STA, &conf) == ESP_OK;