sstables: Enforce estimated_partitions in generate_summary() to be always positive.
sstables: Enforce estimated_partitions in generate_summary() to be always positive.

For tiny index files (< 8 bytes long) it could turn to zero and trigger
an assertion in prepare_summary().

Signed-off-by: Vladimir Krivopalov <dc9186a06078733915a6fcbab34e59120be2b484@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla