prevent 0-length buffers reaching imagemagick
prevent 0-length buffers reaching imagemagick

im6 seems to have added an assert for this

see https://github.com/libvips/libvips/pull/1785

jcupitt/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips