streaming: Check if _stream_result is valid
streaming: Check if _stream_result is valid

If on_error() was called before init() was executed, the
_stream_result can be invalid.

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