check_disk: fix ignore-missing in combination with includes (fixes #1963)

Using --ignore-missing together with regex matching and ignore option lead
to a wrong error message.

    ./check_disk -n -w 10% -c 5% -W 10% -r /dummy -i /dummy2
    DISK UNKNOWN: Paths need to be selected before using -i/-I. Use -A to select all paths explicitly

The use case here is a cluster with fail-over mounts. So it is a valid situation that
the regex match does not find anything in addtition with a ignore which also does not exist.

After this patch:

    ./check_disk -n -w 10% -c 5% -W 10% -r /dummy -i /dummy2
    DISK OK - No disks were found for provided parameters|

0 files changed, 0 insertions, 0 deletions