dma: exit if invoked with invalid (zero) argc

[emaste]

Summary

Prompted by the recent pkexec vulnerability exit with an error if invoked with argc == 0.

[markj]

Apparently basename(NULL) returns ".", I didn't know that.

[kevans]

As a general principle, I think it's still a good idea to audit base stuff for argc == 0 issues. We have downstreams for userland-only stuff, so it'd generally be more for their benefit assuming they may not pick up the kernel change (either because they can't for conformance reasons, or whatever).

In D34087#770727, @markj wrote:

Apparently basename(NULL) returns ".", I didn't know that.

That's... really odd.