Mail: get rid of ugly protocol check in ngx_mail_auth_parse(). Instead, use index of argument which holds authentication mechanism name. For IMAP and POP3 it's 0, for SMTP - 1 as SMTP preserves command in first argument to allow pipelining support. While here, add check that we actually have argument holding authentication mechanism name. Currently IMAP has no appropriate checks before calling ngx_mail_auth_parse() which results in possible access of uninitialized memory.