fix "AUTH PLAIN [initial-response]" bug introduced in r1477

src/mail/ngx_mail_handler.c

diff --git a/src/mail/ngx_mail_handler.c b/src/mail/ngx_mail_handler.c
index d4082da..e4985d3 100644
--- a/src/mail/ngx_mail_handler.c
+++ b/src/mail/ngx_mail_handler.c
@@ -295,7 +295,7 @@
         return NGX_ERROR;
     }
 
-    if (ngx_decode_base64(&plain, &arg[0]) != NGX_OK) {
+    if (ngx_decode_base64(&plain, &arg[n]) != NGX_OK) {
         ngx_log_error(NGX_LOG_INFO, c->log, 0,
             "client sent invalid base64 encoding in AUTH PLAIN command");
         return NGX_MAIL_PARSE_INVALID_COMMAND;