Mail: added a check for the number of arguments in MAIL/RCPT.

Missed during introduction of the SMTP pipelining support (04e43d03e153,
1.5.6).  Previously, the check wasn't needed as s->buffer was used directly
and the number of arguments didn't matter.

Reported by Svyatoslav Nikolsky.

src/mail/ngx_mail_smtp_handler.c

diff --git a/src/mail/ngx_mail_smtp_handler.c b/src/mail/ngx_mail_smtp_handler.c
index 52fe475..665f5ef 100644
--- a/src/mail/ngx_mail_smtp_handler.c
+++ b/src/mail/ngx_mail_smtp_handler.c
@@ -679,6 +679,11 @@
         return NGX_OK;
     }
 
+    if (s->args.nelts == 0) {
+        ngx_str_set(&s->out, smtp_invalid_argument);
+        return NGX_OK;
+    }
+
     arg = s->args.elts;
     arg += s->args.nelts - 1;
 
@@ -713,6 +718,11 @@
         return NGX_OK;
     }
 
+    if (s->args.nelts == 0) {
+        ngx_str_set(&s->out, smtp_invalid_argument);
+        return NGX_OK;
+    }
+
     arg = s->args.elts;
     arg += s->args.nelts - 1;