managesieve-login: managesieve_client_input_next_cmd(): Fix handling of command continuation.

Commands that continued after waiting for more input always failed.
This mainly applies to the AUTHENTICATE command, which mostly handles its own arguments.
This caused problems when a string literal was used as the initial response for the AUTHENTICATE command.

Caused by recent change:

Author: Timo Sirainen <timo.sirainen@dovecot.fi>
Date:   Wed Jun 14 12:39:02 2017 +0300

managesieve-login: managesieve_client_input_next_cmd() - minor code cleanup