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.c

@@ -396,6 +396,35 @@ disco_items_without_name(void **state)
     prof_timeout_reset();
 }
 
+void
+disco_info_result_no_from(void **state)
+{
+    /*
+     * Test that a disco#info result without a 'from' attribute is treated as
+     * coming from the server itself (RFC 6120 §8.1.2.1). The on-connect
+     * disco#info handler used to crash on such responses (issue #168).
+     */
+    stbbr_for_query("http://jabber.org/protocol/disco#info",
+        "<iq to='stabber@localhost/profanity' type='result'>"
+            "<query xmlns='http://jabber.org/protocol/disco#info'>"
+                "<identity category='server' type='im' name='NoFromServer'/>"
+                "<feature var='urn:xmpp:ping'/>"
+            "</query>"
+        "</iq>"
+    );
+
+    /* the on-connect disco#info gets the same from-less response */
+    prof_connect();
+
+    prof_input("/disco info");
+
+    prof_timeout(10);
+    /* client survived and attributed the response to the server */
+    assert_true(prof_output_exact("Service discovery info for localhost"));
+    assert_true(prof_output_regex("NoFromServer.*im.*server"));
+    prof_timeout_reset();
+}
+
 void
 disco_info_service_unavailable(void **state)
 {