config: avoid "write_in_full(fd, buf, len) != len" pattern

config: avoid "write_in_full(fd, buf, len) != len" pattern

As explained in commit 06f46f237 (avoid "write_in_full(fd, buf, len)
!= len" pattern, 2017–09–13) the return value of write_in_full() is
either -1 or the requested number of bytes. As such comparing the
return value to an unsigned value such as strbuf.len will fail to
catch errors. Change the code to use the preferred '< 0' check.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>