shell: clear popup grab interface on remove_popup_grab and popup_grab_end
shell: clear popup grab interface on remove_popup_grab and popup_grab_end

Fixes a segfault. Steps to reproduce:

* start weston with the x11 backend
* open a terminal
* click on the icon in the top left corner, choose close
* close the x11 window containing weston

udoprog/weston,kwm81/weston,eyolfson/weston,mchalupa/weston,Gnurou/weston,udoprog/weston,Tarnyko/weston-xdg_surface_present,krezovic/weston,sir-murray/weston,Tarnyko/weston-xdg_surface_present,xorgy/weston,jonnylamb/weston,kwm81/weston,jonnylamb/weston,Gnurou/weston,eyolfson/weston,xorgy/weston,Fantu/compositor-spice,Fantu/compositor-spice,giucam/weston,giucam/weston,mchalupa/weston,sir-murray/weston,Fantu/compositor-spice,krezovic/weston