Mail: fixed handling of invalid SMTP commands split between reads.

Mail: fixed handling of invalid SMTP commands split between reads.

Previously, if an invalid SMTP command was split between reads, nginx failed
to wait for LF before returning an error, and interpreted the rest of the
command received later as a separate command.

The sw_invalid state in ngx_mail_smtp_parse_command(), introduced in
04e43d03e153, did not work, since ngx_mail_smtp_auth_state() clears
s->state when returning an error due to NGX_MAIL_PARSE_INVALID_COMMAND.
And not clearing s->state will introduce another problem: the rest
of the command would trigger duplicate error when rest of the command is
received.

Fix is to return NGX_AGAIN from ngx_mail_smtp_parse_command() until full
command is received.