pgsql: Fix text substring search for non-deterministic collations.

Fix text substring search for non-deterministic collations.

Due to an off-by-one error, the code failed to find matches at the
end of the haystack.  Fix by rewriting the loop.

While at it, fix a comment that claimed that the function could find
a zero-length match.  Such a match could send a caller into an endless
loop.  However, zero-length matches only make sense with an empty
search string, and that case is explicitly excluded by all callers.
To make sure it stays that way, add an Assert and a comment.

Bug: #19341
Reported-by: Adam Warland <adam.warland@infor.com>
Author: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/19341-1d9a22915edfec58@postgresql.org
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/18b349315ae7e00261732f514d6c91d713bb77d0

Modified Files
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src/backend/utils/adt/varlena.c                | 25 ++++++++++++++++++-------
src/test/regress/expected/collate.icu.utf8.out |  7 +++++++
src/test/regress/sql/collate.icu.utf8.sql      |  3 +++
3 files changed, 28 insertions(+), 7 deletions(-)