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

contrib/dma/dma.c

@@ -428,6 +428,9 @@ main(int argc, char **argv)
 	int nodot = 0, showq = 0, queue_only = 0, newaliases = 0;
 	int recp_from_header = 0;
 
+	if (argc == 0)
+		errx(EX_OSERR, "invalid argc");
+
 	set_username();
 
 	/*