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

src/xmpp/connection.c

@@ -759,11 +759,11 @@ _get_from_via_jid(const char* const jid)
 void
 connection_features_received(const char* const jid)
 {
-    log_info("[CONNECTION] connection_features_received %s", jid);
     const char* key = _get_from_via_jid(jid);
     if (!key) {
         return;
     }
+    log_info("[CONNECTION] connection_features_received %s", key);
     if (g_hash_table_remove(conn.requested_features, key) && g_hash_table_size(conn.requested_features) == 0) {
         sv_ev_connection_features_received();
     }