dma: exit if invoked with invalid (zero) argc

This was prompted by the recent pkexec vulnerability (CVE-2021-4034).
This change is being made on general principle for setuid/setgid
binaries and is not in response to an actual issue.

Reviewed by: kevans, markj (both earlier)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34087

(cherry picked from commit 1c91aedf25bce879a8aced6439200e46601eb8a4)