common: Initialize message in cockpit_web_request_on_socket_input()

g_socket_receive_message() can fail, and a lot of exit paths don't
initialize `ccm.n_messages`.

Initialize the struct (like in test-jsonfds.c), so that
cockpit_control_messages_clear() does not try to free uninitialized
message pointers.

Fixes cockpit-project#17492

src/common/cockpitwebserver.c

@@ -1142,7 +1142,7 @@ cockpit_web_request_on_socket_input (GSocket *socket,
   GError *error = NULL;
   GIOStream *tls_stream;
   gssize num_read;
-  g_auto(CockpitControlMessages) ccm;
+  g_auto(CockpitControlMessages) ccm = COCKPIT_CONTROL_MESSAGES_INIT;
 
   num_read = g_socket_receive_message (socket,
                                        NULL, /* out GSocketAddress */