sstables: In mp_row_consumer_m::push_mutation_fragments(), return the called helper's value.
sstables: In mp_row_consumer_m::push_mutation_fragments(), return the called helper's value.

Instead of blindly proceeding, use whatever the call to maybe_push_*()
has returned.

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

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