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