Fix uninitted class variables from esp8266#7464 (esp8266#7478)

PR esp8266#7464 removed the reset of client authentication settings when server
authentication settings were changed, however it never did initialize
the client authentication information to nullptr in the constructor.
This can result in crashes during connections when client certs are not
applied.

Fix by resetting the client authenticaion variables on object
construction.

libraries/ESP8266WiFi/src/WiFiClientSecureBearSSL.cpp

@@ -105,6 +105,9 @@ WiFiClientSecure::WiFiClientSecure() : WiFiClient() {
   _clear();
   _clearAuthenticationSettings();
   _certStore = nullptr; // Don't want to remove cert store on a clear, should be long lived
+  _sk = nullptr;
+  _axtls_chain = nullptr;
+  _axtls_sk = nullptr;
   stack_thunk_add_ref();
 }