traverse_trees(): use stack array for name entries

We heap-allocate our arrays of name_entry structs, etc, with one entry
per tree we're asked to traverse. The code does a raw multiplication in
the xmalloc() call, which I find when auditing for integer overflows
during allocation.

We could "fix" this by using ALLOC_ARRAY() instead. But as it turns out,
the maximum size of these arrays is limited at compile time:

  - merge_trees() always passes in 3 trees

  - unpack_trees() and its brethren never pass in more than
    MAX_UNPACK_TREES

So we can simplify even further by just using a stack array and bounding
it with MAX_UNPACK_TREES. There should be no concern with overflowing
the stack, since MAX_UNPACK_TREES is only 8 and the structs themselves
are small.

Note that since we're replacing xcalloc(), we have to move one of the
NULL initializations into a loop.

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

tree-walk.c

@@ -410,15 +410,20 @@ int traverse_trees(struct index_state *istate,
 		   struct traverse_info *info)
 {
 	int error = 0;
-	struct name_entry *entry = xmalloc(n*sizeof(*entry));
+	struct name_entry entry[MAX_UNPACK_TREES];
 	int i;
-	struct tree_desc_x *tx = xcalloc(n, sizeof(*tx));
+	struct tree_desc_x tx[ARRAY_SIZE(entry)];
 	struct strbuf base = STRBUF_INIT;
 	int interesting = 1;
 	char *traverse_path;
 
-	for (i = 0; i < n; i++)
+	if (n >= ARRAY_SIZE(entry))
+		BUG("traverse_trees() called with too many trees (%d)", n);
+
+	for (i = 0; i < n; i++) {
 		tx[i].d = t[i];
+		tx[i].skip = NULL;
+	}
 
 	if (info->prev) {
 		strbuf_make_traverse_path(&base, info->prev,
@@ -506,10 +511,8 @@ int traverse_trees(struct index_state *istate,
 			if (mask & (1ul << i))
 				update_extended_entry(tx + i, entry + i);
 	}
-	free(entry);
 	for (i = 0; i < n; i++)
 		free_extended_entry(tx + i);
-	free(tx);
 	free(traverse_path);
 	info->traverse_path = NULL;
 	strbuf_release(&base);