exp/locale/collate/build: fixed bug that was exposed by experimenting

exp/locale/collate/build: fixed bug that was exposed by experimenting
with table changes.
NOTE: there is no test for this, but 1) the code has now the same
control flow as scan in exp/locale/collate/contract.go, which is
tested and 2) Builder verifies the generated table so bugs in this
code are quickly and easily found (which is how this bug was discovered).

R=r
CC=golang-dev
https://golang.org/cl/6461082