common: spice_memdup could accept NULL
common: spice_memdup could accept NULL

(this patch is not to solve a crash fix, but to align with glib API)

dezelin/spice,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,SPICE/spice,Fantu/spice,fgouget/spice,Fantu/spice,elmarco/spice,ya-mouse/spice,dezelin/spice,alon/spice-server,Fantu/spice,dezelin/spice,fgouget/spice,alon/spice-server,alon/spice-server,ya-mouse/spice,ya-mouse/spice,fgouget/spice,mathslinux/spice,elmarco/spice,mathslinux/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,elmarco/spice,SPICE/spice,elmarco/spice,dezelin/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,dezelin/spice,elmarco/spice,Fantu/spice,mathslinux/spice,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,mathslinux/spice,alon/spice-server