[gdb] Make variable printed bool in info_checkpoints_command

While reading info_checkpoints_command, I noticed variable printed:
...
  const fork_info *printed = NULL;
  ...
  for (const fork_info &fi : fork_list)
    {
      if (requested > 0 && fi.num != requested)
	continue;

      printed = &fi;
      ...
    }
  if (printed == NULL)
...
has pointer type, but is just used as bool.

Make this explicit by changing the variable type to bool.

Tested on x86_64-linux.

Approved-By: Kevin Buettner <kevinb@redhat.com>