colblk: fix UintsBuilder bug in default mode

[RaduBerinde]

When we are in useDefault mode, the slice can be smaller than the
number of rows. In that case, the existing code was reading outside
the underlying slice and writing out garbage.

We should reconsider the use of UnsafeSlice here, as I don't think
it's buying us much performance-wise.

Informs cockroachdb/cockroach#132964

[cockroach-teamcity]

This change is 

[RaduBerinde]

It's worth noting that TestCockroachDataBlock finds this issue on master (now that we also generate some logical times):

go test -tags invariants --exec 'stress -p 4' -timeout 0 -v ./sstable/colblk -run CockroachDataBlock

=== RUN   TestCockroachDataBlock
    cockroach_test.go:27: seed: 1729349928313842000
--- FAIL: TestCockroachDataBlock (0.00s)
panic: incorrect target width [recovered]
        panic: incorrect target width

The test passes reliably with the fix.

[jbowens]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RaduBerinde)

[jbowens]

Good catch!

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @RaduBerinde)

[RaduBerinde]

TFTR!