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During precalculation and in step_two, we begin looking for an end
match only after the full start match, not merely one byte beyond
its starting point.  So do that too when searching backward for an
unpaired start match.

Also, index can never be zero here, because if the match was of length
zero, this piece of code will have been skipped by the preceding goto.
So we can always use REG_NOTBOL here.

(That goto is wrong, by the way: https://savannah.gnu.org/bugs/?50078,
but that will follow later.)