initialize io_ctx to nullptr

Author: ztittle

libraries/ESP8266WiFi/src/WiFiClientSecure.cpp

@@ -420,7 +420,7 @@ class SSLContext
     size_t _available = 0;
     BufferList _writeBuffers;
     bool _allowSelfSignedCerts = false;
-    ClientContext* io_ctx;
+    ClientContext* io_ctx = nullptr;
 };
 
 SSL_CTX* SSLContext::_ssl_ctx = nullptr;