btrfs-progs: qgroup-verify: avoid NULL pointer dereference for later silent qgroup repair

Commit 078e9a1cc9 ("btrfs-progs: check: enhanced progress indicator")
introduced @qgroup_item_count for progress indicator.

However since we will later introduce silent qgroup rescan
functionality, the @qgroup_item_count pointer can be NULL.

So check if @qgroup_item_count is NULL before accessing it.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

check/qgroup-verify.c

@@ -743,7 +743,8 @@ static int travel_tree(struct btrfs_fs_info *info, struct btrfs_root *root,
 	 */
 	nr = btrfs_header_nritems(eb);
 	for (i = 0; i < nr; i++) {
-		(*qgroup_item_count)++;
+		if (qgroup_item_count)
+			(*qgroup_item_count)++;
 		new_bytenr = btrfs_node_blockptr(eb, i);
 		new_num_bytes = info->nodesize;