ESP8266WiFi: zero-initialise scan_config structure

Fixes #4394

Author: igrr

libraries/ESP8266WiFi/src/ESP8266WiFiScan.cpp

@@ -83,9 +83,7 @@ int8_t ESP8266WiFiScanClass::scanNetworks(bool async, bool show_hidden) {
     scanDelete();
 
     struct scan_config config;
-    config.ssid = 0;
-    config.bssid = 0;
-    config.channel = 0;
+    memset(&config, 0, sizeof(config));
     config.show_hidden = show_hidden;
     if(wifi_station_scan(&config, reinterpret_cast<scan_done_cb_t>(&ESP8266WiFiScanClass::_scanDone))) {
         ESP8266WiFiScanClass::_scanComplete = false;