convert trivial cases to ALLOC_ARRAY

Each of these cases can be converted to use ALLOC_ARRAY or
REALLOC_ARRAY, which has two advantages:

  1. It automatically checks the array-size multiplication
     for overflow.

  2. It always uses sizeof(*array) for the element-size,
     so that it can never go out of sync with the declared
     type of the array.

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

column.c

@@ -164,7 +164,7 @@ static void display_table(const struct string_list *list,
 	data.colopts = colopts;
 	data.opts = *opts;
 
-	data.len = xmalloc(sizeof(*data.len) * list->nr);
+	ALLOC_ARRAY(data.len, list->nr);
 	for (i = 0; i < list->nr; i++)
 		data.len[i] = item_length(colopts, list->items[i].string);