libblkid: fix infinite loop when probe chain bails out early

libblkid: fix infinite loop when probe chain bails out early

The superblocks probe bails out early with no results in some cases.  If
this happens, blkid_do_probe needs to go to the next chain, rather than
entering an infinite loop calling superblocks_probe over and over again.

[kzak@redhat.com: - print debug message always when leaving
                    superblocks_probe()]

Addresses: https://bugs.launchpad.net/bugs/528073
Signed-off-by: Colin Watson <cjwatson@canonical.com>
Signed-off-by: Karel Zak <kzak@redhat.com>