shell: Keep shsurf->fullscreen_output set after unset_fullscreen()

When a fullscreen surface gets the maximized state, the function
reset_surface_type() is called and that causes unset_fullscreen() to be
called. That function would set the value of shsurf->fullscreen_output
to NULL. However, since the surface still has the fullscreen state, it
will be configured as a fullscreen surface again, and an attempt to
access that field would cause the compositor to crash.

Fix the crash by keeping the value of fullscreen_output around after
unset_fullscreen(). This is safe since the value is only used when a
surface has the fullscreen state and is replaced on a new request to
make the surface fullscreen.

https://bugs.freedesktop.org/show_bug.cgi?id=76867

desktop-shell/shell.c

@@ -2340,7 +2340,6 @@ unset_fullscreen(struct shell_surface *shsurf)
 	    shell_surface_is_top_fullscreen(shsurf)) {
 		restore_output_mode(shsurf->fullscreen_output);
 	}
-	shsurf->fullscreen_output = NULL;
 
 	shsurf->fullscreen.type = WL_SHELL_SURFACE_FULLSCREEN_METHOD_DEFAULT;
 	shsurf->fullscreen.framerate = 0;