fix a used-before-set error in im_vips2dz
fix a used-before-set error in im_vips2dz

we were reading an uninited string in a vips7 compatibility wrapper, thanks
yifengchen-cc

see https://github.com/libvips/libvips/issues/1419

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