Fix YA text phrase search bug.

Fix YA text phrase search bug.

checkcondition_str() failed to report multiple matches for a prefix
pattern correctly: it would dutifully merge the match positions, but
then after exiting that loop, if the last prefix-matching word had
had no suitable positions, it would report there were no matches.
The upshot would be failing to recognize a match that the query
should match.

It looks like you need all of these conditions to see the bug:
* a phrase search (else we don't ask for match position details)
* a prefix search item (else we don't get to this code)
* a weight restriction (else checkclass_str won't fail)

Noted while investigating a problem report from Pavel Borisov,
though this is distinct from the issue he was on about.

Back-patch to 9.6 where phrase search was added.