midx-write: simplify error cases

midx-write: simplify error cases

The write_midx_internal() method uses gotos to jump to a cleanup section to
clear memory before returning 'result'. Since these jumps are more common
for error conditions, initialize 'result' to -1 and then only set it to 0
before returning with success. There are a couple places where we return
with success via a jump.

This has the added benefit that the method now returns -1 on error instead
of an inconsistent 1 or -1.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>