Forbid headers with only a region

[DemiMarie]

Such headers are useless and might cause problems elsewhere.

[pmatilai]

It's entirely legitimate for a header to be empty, region or no.

A package cannot have an empty header (or region), but that'd be a different kind of check.

[DemiMarie]

We actually already reject empty headers (with no tags at all) in hdrblobVerifyRegion. So the only question remaining is if an empty region (ril == 0) is valid.

[pmatilai]

Oh, right, I'd forgotten about that. That existing check is wrong in it's current habitat, it got moved around in the last big refactoring. A header is fundamentally nothing but an abstract data container, and being empty is an entirely valid condition for it to be in.

[DemiMarie]

What about checking that a header/region is non-empty if (and only if) the regionTag passed to hdrblobVerifyRegion is non-zero?