libblkid: zeroize errno on blkid_probe_get_buffer() success

Since 37f4060225df0591ab8e1dd676dbc8115d900d4f prober functions are
sensitive to errno, it seems more robust to set errno=0 with in
blkid_probe_get_buffer() on success than set the zero on all places
where we call blkid_probe_get_buffer().

Addresses: https://github.com/karelzak/util-linux/issues/119
Signed-off-by: Karel Zak <kzak@redhat.com>