Solve issue #2092 by initializing * _client to nullptr (#2097)

Author: Jeroen88

Diff for: libraries/HTTPClient/src/HTTPClient.h

@@ -208,7 +208,7 @@ class HTTPClient
     std::unique_ptr<WiFiClient> _tcpDeprecated;
 #endif
 
-    WiFiClient* _client;
+    WiFiClient* _client = nullptr;
 
     /// request handling
     String _host;