add_reflog_for_walk: avoid memory leak

add_reflog_for_walk: avoid memory leak

We free()d the `log` buffer when dwim_log() returned 1, but not when it
returned a larger value (which meant that it still allocated the buffer
but we simply ignored it).

While in the vicinity, make sure that the `reflogs` structure as well as
the `branch` variable are released properly, too.

Identified by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>