Fix when feature discovery is finished

Feature discovery was marked as finished once we received a reply to the
initial request. The discovery mechanism allows to delegate the real
feature discovery to another service running on different domain and those
requests are created dynamically.

This was another instance causing the warning message described in #1940

Fixes #1940 (once more)

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

tests/unittests/xmpp/stub_xmpp.c

@@ -450,6 +450,11 @@ iq_mam_request(ProfChatWin* win, GDateTime* enddate)
 {
 }
 
+void
+iq_feature_retrieval_complete_handler(void)
+{
+}
+
 void
 publish_user_mood(const char* const mood, const char* const text)
 {