Read up remaining lines in assuan_inquire after reaching MAXLEN

* src/assuan-inquire.c (assuan_inquire): Clear return args on error.
Read up remaining lines after MAXLEN has been hit.

If we would stop immediately at MAXLEN, as we did, the client would
continue to send data which the server may consider as new Assuan
commands.

• Signed-off-by: Werner Koch <wk@gnupg.org>