buffer: do not append trailing newline when appending empty istream

buffer: do not append trailing newline when appending empty istream

If we call

 bl.append(some_istream);

do not include a \n if the istream is empty (which is apparently is not
the same thing as eof).  This was causing 'ceph pg getmap' to include a
trailing newline.

Probably we don't want this newline at all!  But all callers need to be
fixed for that change.

Signed-off-by: Sage Weil <sage@inktank.com>