Reported in https://github.com/esp8266/Arduino/issues/4078.
WiFiClient::stopAll, called from a WiFi disconnected event handler,
could be called while WiFiClient::connect was in progress. This issue
was initially fixed in #4194, by testing `this` pointer for being
non-null in ClientContext::connect.
This change delegates deletion of ClientContext to WiFiClient
destructor. WiFiClient::stop only calls ClientContext::stop, which
closes/aborts the connection.
