unicode/norm: let any starter block composition in compose

reorderBuffer.compose only refreshed s, the index of the last starter,
inside the ii.combinesBackward() branch. A rune that does not combine
backward therefore left s pointing at whatever starter preceded it, and
the next backward-combining mark composed with that stale starter even
though an intervening starter should have blocked it.

For example, NFKC of U+16D68 U+FB01 U+113C2 U+0300 U+0316 decomposes to
U+16D68 f i U+113C2 U+0316 U+0300. U+0316 does not combine backward, so
s was left pointing at the "i" rather than at the starter U+113C2, and
U+0300 composed with the "i" to give U+00EC, dropping a code point and
producing a string that is not canonically equivalent to the input.

Update s on every iteration, as combineHangul already does.

Fixes golang/go#81001.

Change-Id: I4d2332b4a0c44d60bd0961036391617a728c12df
Reviewed-on: https://go-review.googlesource.com/c/text/+/822081
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>