Minor bug: 520-byte limit was not set for witness 0

[dgpv]

Due to trivial error, the 520-byte limit was not set for witness 0 when first accessing it. This limit will be usually set in other places, too, so it is unclear if this would ever surface as a problem.

This is a minor bug, but a bug nevertheless.

Fixed in https://github.com/dgpv/bsst/commit/518ebaa765b5d95b8df9157bfbeaa7bcb0c80010