Fix a crash on zxdg_decoration_manager_v1 destroy
Fix a crash on zxdg_decoration_manager_v1 destroy

When this request was called from a client, the unimplemented callback
was NULL and thus was crashing rootston.

ascent12/wlroots,ascent12/wlroots,SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots