sstables: fix probe with Unknown component
sstables: fix probe with Unknown component

Commit 53b7b7def3a95456ef81 ("sstables: handle unrecognized sstable component")
ignores unrecognized components, but misses one code path during probe_file().

Ignore unrecognized components there too.

Fixes #1922.
Message-Id: <d045d0e10e2ddd4859105e0d7911e571d8853a76@scylladb.com>

duarten/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla