reflog-walk: include all fields when freeing complete_reflogs

reflog-walk: include all fields when freeing complete_reflogs

When we encounter an error adding reflogs for a walk, we try
to free any logs we have read. But we didn't free all
fields, meaning that we could in theory leak all of the
"items" array (which would consitute the bulk of the
allocated memory).

This patch adds a helper which frees all of the entries and
uses it as appropriate.

As it turns out, the leak seems impossible to trigger with
the current code. Of the three error paths that free the
complete_reflogs struct, two only kick in when the items
array is empty, and the third was removed entirely in the
previous commit.

So this patch should be a noop in terms of behavior, but it
fixes a potential maintenance headache should anybody add a
new error path and copy the partial-free code. Which is
what happened in 5026b47175 (add_reflog_for_walk: avoid
memory leak, 2017-05-04), though its leaky call was the
third one that was recently removed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>