sstables: Convert an unreachable throw into an assert in read path
sstables: Convert an unreachable throw into an assert in read path

The function pending_collection is only called when
cdef->is_multi_cell() is true, so the throw is dead.

This patch converts it to an assert.
Message-Id: <620fc73bd1f874231a3d5336ef612b7702dfa4b3@scylladb.com>

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