mutation_partition: correctly measure static row size when doing digest calculation
mutation_partition: correctly measure static row size when doing digest calculation

The code uses incorrect output stream in case only digest is requested
and thus getting incorrect data size. Failing to correctly account
for static row size while calculating digest may cause digest mismatch
between digest and data query.

Fixes #3753.

Message-Id: <ecb398663a276e6a2ca6d51189f9e85bcbb09087@scylladb.com>

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