fix void* arithmetic
fix void* arithmetic

marshaller.c:528:50: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]

Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,SPICE/spice-common,elmarco/spice-common,freedesktop-unofficial-mirror/spice__spice-common,fgouget/spice-common,fgouget/spice-common,Fantu/spice-common,elmarco/spice-common,Fantu/spice-common,freedesktop-unofficial-mirror/spice__spice-common,Fantu/spice-common,fgouget/spice-common,SPICE/spice-common,freedesktop-unofficial-mirror/spice__spice-common,fgouget/spice-common,elmarco/spice-common,elmarco/spice-common,SPICE/spice-common