zero all new memory from allocations

Change my_alloc() to use calloc instead of malloc so all fresh
allocations return zeroed memory. Also zero the expanded portion
in expand_item_list() after realloc, since it knows both old and
new sizes. This gives more predictable behaviour in case of bugs
where uninitialised or stale memory is accidentally accessed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

util1.c

@@ -1718,6 +1718,8 @@ void *expand_item_list(item_list *lp, size_t item_size, const char *desc, int in
 				new_ptr == lp->items ? " not" : "");
 		}
 
+		memset((char *)new_ptr + lp->malloced * item_size, 0,
+		       (expand_size - lp->malloced) * item_size);
 		lp->items = new_ptr;
 		lp->malloced = expand_size;
 	}

util2.c

@@ -79,9 +79,7 @@ void *my_alloc(void *ptr, size_t num, size_t size, const char *file, int line)
 			who_am_i(), do_big_num(max_alloc, 0, NULL), src_file(file), line);
 		exit_cleanup(RERR_MALLOC);
 	}
-	if (!ptr)
-		ptr = malloc(num * size);
-	else if (ptr == do_calloc)
+	if (!ptr || ptr == do_calloc)
 		ptr = calloc(num, size);
 	else
 		ptr = realloc(ptr, num * size);