Fix an error when io.CopyBuffer returns n=0

It could be that the "n=0 means EOF" semantics are leaking from the
underlying splice() system call? Anyway this prevents radiod from
getting stuck in a busy loop in certain(?) cases.
3 jobs for v2 in 4 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build Pkgsrc
passed #7402
build_pkgsrc:stretch

00:00:16

 
  Build Pkg
passed #7403
amd64
build_pkg:stretch:amd64

00:01:19

passed #7404
arm64
build_pkg:stretch:arm64

00:04:22