GSocket: Properly initialize msg.msg_control

GSocket: Properly initialize msg.msg_control

This patch fixes this problem

   Syscall param socketcall.sendmsg(msg.msg_control) points to uninitialised byte(s)
      at 0x3D5B00EA60: __sendmsg_nocancel (syscall-template.S:82)
      by 0x53F9790: g_socket_send_message (gsocket.c:2918)
      by 0x540FDD0: g_unix_connection_send_credentials (gunixconnection.c:351)
      by 0x542B93F: _g_dbus_auth_run_client (gdbusauth.c:618)
      by 0x5438001: initable_init (gdbusconnection.c:2191)
      by 0x53E09CC: g_initable_init (ginitable.c:105)
      by 0x543F6E9: g_bus_get_sync (gdbusconnection.c:6091)
      by 0x402C7E: test_connection_life_cycle (gdbus-connection.c:126)
      by 0x4C7CABB: test_case_run (gtestutils.c:1174)
      by 0x4C7CD84: g_test_run_suite_internal (gtestutils.c:1223)
      by 0x4C7CE49: g_test_run_suite_internal (gtestutils.c:1233)
      by 0x4C7CE49: g_test_run_suite_internal (gtestutils.c:1233)
    Address 0x7fefff9fc is on thread 1's stack

Signed-off-by: David Zeuthen <davidz@redhat.com>