ssd: Fix segfault when disabling output with fullscreen view

ssd: Fix segfault when disabling output with fullscreen view

wlr_output_layout_get() returns NULL for disabled outputs.

Backtrace (abbreviated):

     #0  lab_wlr_output_layout_layout_coords at ../src/ssd/ssd_extents.c:33
     #1  ssd_extents_update at ../src/ssd/ssd_extents.c:133
     #2  ssd_extents_create at ../src/ssd/ssd_extents.c:95
     #3  ssd_create at ../src/ssd/ssd.c:161
     #4  decorate.part.0.lto_priv.0 at ../src/view.c:637
     #5  decorate at ../src/view.c:636
     #6  view_set_fullscreen at ../src/view.c:716
     #7  view_adjust_for_layout_change ../src/view.c:745
     #8  desktop_arrange_all_views ../src/desktop.c:52
     #9  output_update_all_usable_areas at ../src/output.c:495
     #10 output_update_for_layout_change at ../src/output.c:263
     #11 do_output_layout_change at ../src/output.c:423
     #12 do_output_layout_change at ../src/cursor.c:267
     #13 output_config_apply at ../src/output.c:334
     #14 handle_output_manager_apply at ../src/output.c:354