Allow NULL out_message_finished in stream_membuf_read
Allow NULL out_message_finished in stream_membuf_read

Summary: LibCWMP does not care about the flag when it calls `avs_stream_getch`, so it passes NULL there.

Test Plan: libcwmp make check

Reviewers: kfyatek, mkrawiec

Differential Revision: https://phabricator.avsystem.com/D3372

AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons