rerere: avoid buffer overrun

check_one_conflict() compares `i` to `active_nr` in two places to avoid buffer overruns, but left out an important third location. The code did used to have a check here comparing i to active_nr, back before commit fb70a06da2f1 ("rerere: fix an off-by-one non-bug", 2015-06-28), however the code at the time used an 'if' rather than a 'while' meaning back then that this loop could not have read past the end of the array, making the check unnecessary and it was removed. Unfortunately, in commit 5eda906b2873 ("rerere: handle conflicts with multiple stage #1 entries", 2015-07-24), the 'if' was changed to a 'while' and the check comparing i and active_nr was not re-instated, leading to this problem. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>