strbuf: release memory on read error in strbuf_read_once()

strbuf: release memory on read error in strbuf_read_once()

If other strbuf add functions cause the first allocation and
subsequently encounter an error then they release the memory, restoring
the pristine state of the strbuf.  That simplifies error handling for
callers.

Do the same in strbuf_read_once(), and do it also in case no bytes were
read -- which may or may not be an error as well, depending on the
caller.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>