desktop-shell: NULL check whether a popup parent is a shell surface
desktop-shell: NULL check whether a popup parent is a shell surface

get_shell_surface(parent) may return NULL if the client passed a
unassigned wl_surface or a wl_surface with a non-shell surface role
(such as cursor role).

https://bugs.freedesktop.org/show_bug.cgi?id=92316

Signed-off-by: Jonas Ådahl <67dcd8a17e758192e0cbb577ce1d62cc7da6cf49@gmail.com>
Tested-by: Olivier Fourdan <2c4ac03450ebb386477a623524ea7610d7285173@redhat.com>
Reviewed-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>

Fantu/compositor-spice,sir-murray/weston,kwm81/weston,Fantu/compositor-spice,krezovic/weston,Gnurou/weston,Fantu/compositor-spice,kwm81/weston,giucam/weston,Gnurou/weston,krezovic/weston,sir-murray/weston,giucam/weston