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.