Mail: fixed possible uninitialized memory access.

Found by Valgrind.

src/mail/ngx_mail_parse.c

diff --git a/src/mail/ngx_mail_parse.c b/src/mail/ngx_mail_parse.c
index eb16d5b..7de6c19 100644
--- a/src/mail/ngx_mail_parse.c
+++ b/src/mail/ngx_mail_parse.c
@@ -842,6 +842,10 @@
     }
 #endif
 
+    if (s->args.nelts == 0) {
+        return NGX_MAIL_PARSE_INVALID_COMMAND;
+    }
+
     arg = s->args.elts;
 
     if (arg[0].len == 5) {