A zero-length copy_stream fell through the cracks along the non-IO
path, failing to detect that nothing should be transferred and
proceeding to copy the maximum chunk size repeatedly. This caused
jruby#9377 and resulted in zero-length files from gems being
written with the entire contents of the remaining tar stream.

The fix here detects a zero length on the way into each form of
transfer and bails out before any work is done.

Fixes jruby#9377.