pgsql: Fix contrib/ltree's subpath() with negative offset.

Fix contrib/ltree's subpath() with negative offset.

subpath(ltree,offset,len) now correctly errors when given an offset
less than -n, where n is the number of labels in the given ltree.
There was a duplicate block of code that allowed an offset as low
as -2n. The documentation says no such thing, so this must have
been a copy-and-paste error in the original ltree patch.

While here, avoid redundant calculation of "end" and write
LTREE_MAX_LEVELS rather than its hard-coded value.

Author: Marcus Gartner <m(dot)a(dot)gartner(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAAUGV_SvBO9gWYbaejb9nhe-mS9FkNP4QADNTdM3wdRhvLobwA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ff8aba65d463b144db7c081181b5ccf6eaaf1af4

Modified Files
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contrib/ltree/expected/ltree.out | 2 ++
contrib/ltree/ltree_op.c | 14 +++-----------
contrib/ltree/sql/ltree.sql | 1 +
3 files changed, 6 insertions(+), 11 deletions(-)