search: Fix a NULL ptr deref with zero terms

[cgwalters]

Here clang-analyzer found a legitimate bug, through a quite deep call stack. Before this change, providing zero search terms like this results in a segfault:

$ rpmostree_busctl_call_os Search as 0

In exactly the way predicted by the static analysis.

Verify we have at least one term at entry into the function, and also add further assertions later.

[jmarrero]

lgtm

[cgwalters]

/retest
Sigh, two different test flakes

[lukewarmtemp]

@cgwalters Should I update the kola test with a case with zero search terms as well?

[cgwalters]

@cgwalters Should I update the kola test with a case with zero search terms as well?

Yes, arguably I should have done that in this PR too. But we can do that in a followup PR to avoid a whole retest cycle.

[lukewarmtemp]

LGTM. I think I implemented a check on the client side which prevents users from inputting zero packages when running rpm-ostree search, but completely forgot to consider the case when dbus is called directly, so thanks for catching this.