fix(xmpp): treat disco#info result without 'from' as from the server

RFC 6120 §8.1.2.1: a stanza received over a c2s stream without a 'from'
attribute must be treated as coming from the server itself. The
on-connect disco#info handler passed the absent attribute as NULL into
connection_features_received(), where g_str_hash() dereferenced the NULL
key and crashed (remotely triggerable DoS on connect).

Substitute connection_get_domain() at both disco#info handler
boundaries, and make connection_features_received() and
connection_get_features() NULL-safe as defense in depth. Add a stabber
regression test answering the on-connect disco#info with a from-less
result.

Fixes #168

tests/functionaltests/test_disco.h

@@ -17,3 +17,4 @@ void disco_info_multiple_identities(void **state);
 void disco_info_without_name(void **state);
 void disco_items_without_name(void **state);
 void disco_info_service_unavailable(void **state);
+void disco_info_result_no_from(void **state);