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

@@ -749,11 +749,18 @@ connection_get_user(void)
     return connection_get_jid()->localpart;
 }
 
+// NULL 'from' means the server (RFC 6120 §8.1.2.1)
+static const char*
+_get_from_via_jid(const char* const jid)
+{
+    return jid ? jid : conn.domain;
+}
+
 void
 connection_features_received(const char* const jid)
 {
     log_info("[CONNECTION] connection_features_received %s", jid);
-    const char* key = jid ? jid : conn.domain; // g_str_hash crashes on NULL; NULL 'from' means the server (RFC 6120 §8.1.2.1)
+    const char* key = _get_from_via_jid(jid);
     if (!key) {
         return;
     }
@@ -765,7 +772,7 @@ connection_features_received(const char* const jid)
 GHashTable*
 connection_get_features(const char* const jid)
 {
-    const char* key = jid ? jid : conn.domain;
+    const char* key = _get_from_via_jid(jid);
     if (!key || !conn.features_by_jid) {
         return NULL;
     }