We call read_istream, but never check its return value for
errors. This can lead to us looping infinitely, as we just
keep trying to write "-1" bytes (and we do not notice the
error, as we simply check that write_in_full reports the
same number of bytes we fed it, which of course is also -1).
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>