vmctl: off-by-one error handling mixing -a with a VM id

[Preben Guldberg]

Thanks to Dave, Theo and Klemens for accepting my previous patch

In other tests, I noticed the following error:

    % vmctl stop -a testvm
    vmctl: invalid id: (null)

The patch below addresses an off-by-one error reading argv when
generating the error message.

I personally find it clearer if the condition of mixing -a with an id
is highlighted. I included a suggestion in the patch below.

Index: usr.sbin/vmctl/main.c
===================================================================
RCS file: /cvs/src/usr.sbin/vmctl/main.c,v
retrieving revision 1.62
diff -u -p -u -p -r1.62 main.c
--- usr.sbin/vmctl/main.c       3 Jan 2020 05:32:00 -0000       1.62
+++ usr.sbin/vmctl/main.c       12 Mar 2021 11:23:26 -0000
@@ -961,7 +961,7 @@ ctl_stop(struct parse_result *res, int a
        /* VM id is only expected without the -a flag */
        if ((res->action != CMD_STOPALL && ret == -1) ||
            (res->action == CMD_STOPALL && ret != -1))
-               errx(1, "invalid id: %s", argv[1]);
+               errx(1, "invalid use of id with -a: %s", argv[0]);
 
        return (vmmaction(res));
 }