last-modified: fix use of uninitialized memory

git-last-modified(1) uses a scratch bitmap to keep track of paths that
have been changed between commits. To avoid reallocating a bitmap on
each call of process_parent(), the scratch bitmap is kept and reused.
Although, it seems an incorrect length is passed to memset(3).

`struct bitmap` uses `eword_t` to for internal storage. This type is
typedef'd to uint64_t. To fully zero the memory used by the bitmap,
multiply the length (saved in `struct bitmap::word_alloc`) by the size
of `eword_t`.

Reported-by: Anders Kaseorg <andersk@mit.edu>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: To1ne

builtin/last-modified.c

@@ -327,7 +327,7 @@ static void process_parent(struct last_modified *lm,
 	if (!(parent->object.flags & PARENT1))
 		active_paths_free(lm, parent);
 
-	memset(lm->scratch->words, 0x0, lm->scratch->word_alloc);
+	memset(lm->scratch->words, 0x0, lm->scratch->word_alloc * sizeof(eword_t));
 	diff_queue_clear(&diff_queued_diff);
 }