fix an assert fail with libheif 1.10
fix an assert fail with libheif 1.10

if you built heifload with DEBUG it could ask for bpp for channels
which do not exist, triggering an assert fail in libheif 1.10

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