Also change the Version type to avoid similar bugs in the future.

Reported by Grigori Goronzy.

Shrinks the code size by some 2% even.

So we know when the size is static and when dynamic.

Also remove wrong ASSERT_SIZE that would trigger only when not using
flexible arrays (which was the case on win32, but not with gcc).

The major-version check is now handled by sanitize.  If major
doesn't match, we reject and fall back to the Null object.

We handle major-version differences via get_for_data(), so sanitize
should never see a major version mismatch.

That part is performed by individual table sanitize.