window: Don't call drag_offer_handler if it's NULL
window: Don't call drag_offer_handler if it's NULL

Reported by canbaby@21cn.com.

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