screencast: Fix valgrind warning

Fixes the following warning when cancelling a screen cast dialog.

==343654== Invalid read of size 8 ==343654== at 0x48486A: screen_cast_dialog_handle_free (screencast.c:103) ==343654== by 0x4848F9: screen_cast_dialog_handle_close (screencast.c:113) ==343654== by 0x48640D: screen_cast_session_close (screencast.c:916) ==343654== by 0x489BE6: session_close (session.c:87) ==343654== by 0x489C7B: handle_close (session.c:117) ==343654== by 0x410696: _g_dbus_codegen_marshal_BOOLEAN__OBJECT (xdg-desktop-portal-dbus.c:444) ==343654== by 0x436254: xdp_impl_session_method_marshal_close (xdg-desktop-portal-dbus.c:28756) ==343654== by 0x4CF9567: g_closure_invoke (gclosure.c:833) ==343654== by 0x4D0C5FB: signal_emit_unlocked_R.isra.0 (gsignal.c:3928) ==343654== by 0x4D1311A: signal_emitv_unlocked (gsignal.c:3212) ==343654== by 0x4D1311A: g_signal_emitv (gsignal.c:3112) ==343654== by 0x43772F: _xdp_impl_session_skeleton_handle_method_call (xdg-desktop-portal-dbus.c:29485) ==343654== by 0x4C24D21: g_dbus_interface_method_dispatch_helper (gdbusinterfaceskeleton.c:618) ==343654== Address 0x1c5acb10 is 16 bytes inside a block of size 40 free'd ==343654== at 0x4845B2C: free (vg_replace_malloc.c:985) ==343654== by 0x4848DE: screen_cast_dialog_handle_free (screencast.c:107) ==343654== by 0x4848F9: screen_cast_dialog_handle_close (screencast.c:113) ==343654== by 0x484C9C: on_request_handle_close_cb (screencast.c:216) ==343654== by 0x410696: _g_dbus_codegen_marshal_BOOLEAN__OBJECT (xdg-desktop-portal-dbus.c:444) ==343654== by 0x42EE5A: xdp_impl_request_method_marshal_close (xdg-desktop-portal-dbus.c:24008) ==343654== by 0x4CF9567: g_closure_invoke (gclosure.c:833) ==343654== by 0x4D0BFC8: signal_emit_unlocked_R.isra.0 (gsignal.c:3888) ==343654== by 0x4D1311A: signal_emitv_unlocked (gsignal.c:3212) ==343654== by 0x4D1311A: g_signal_emitv (gsignal.c:3112) ==343654== by 0x42FE83: _xdp_impl_request_skeleton_handle_method_call (xdg-desktop-portal-dbus.c:24590) ==343654== by 0x4C24D21: g_dbus_interface_method_dispatch_helper (gdbusinterfaceskeleton.c:618) ==343654== by 0x4C0A837: call_in_idle_cb (gdbusconnection.c:5013)

(cherry picked from commit 9a4dffaf)