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 fb70a06da2 ("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 5eda906b28 ("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>

rerere.c

@@ -532,7 +532,7 @@ static int check_one_conflict(int i, int *type)
 	}
 
 	*type = PUNTED;
-	while (ce_stage(active_cache[i]) == 1)
+	while (i < active_nr && ce_stage(active_cache[i]) == 1)
 		i++;
 
 	/* Only handle regular files with both stages #2 and #3 */

t/t4200-rerere.sh

@@ -599,7 +599,7 @@ test_expect_success 'setup simple stage 1 handling' '
 	)
 '
 
-test_expect_failure 'test simple stage 1 handling' '
+test_expect_success 'test simple stage 1 handling' '
 	(
 		cd stage_1_handling &&