lsblk: ignore device if disappear while processing

lsblk: ignore device if disappear while processing

If lsblk runing on system where many devices appears and disappears
during lsblk run, lsblk should not fail or crash but just ignore
disappeared device.

Let's detect context initialization failures and skip device
instead of failing.

Also fix possible dereferencing of NULL parent pointer and
properly handle some error paths.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>