Prefer g_malloc over vips_malloc in vips_blob_copy (#2489)
Prefer g_malloc over vips_malloc in vips_blob_copy (#2489)

The use of vips_malloc is not necessary here and could potentially
be slower since it uses g_malloc0 (i.e. calloc).
lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips