[commit][obv] Use TYPE_LENGTH directly where possible

[Tom Tromey]

Joel> But I can definitely see someone like myself missing that subtlety,
Joel> and commit an obvious change that reduces the duplication by re-using
Joel> the variable in the gdb_assert call. To the unattentive me, that's
Joel> an obvious improvement. I think that's an issue.

Yeah, that's easy to picture happening.

It seems to me that a comment would be adequate.

Tom