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

contrib/ltree/expected/ltree.out

@@ -128,6 +128,8 @@ SELECT subpath('Top.Child1.Child2',1);
  Child1.Child2
 (1 row)
 
+SELECT subpath('Top.Child1.Child2',-4);  -- error
+ERROR:  invalid positions
 SELECT index('1.2.3.4.5.6','1.2');
  index 
 -------

contrib/ltree/ltree_op.c

@@ -316,23 +316,15 @@ subpath(PG_FUNCTION_ARGS)
 	int32		end;
 	ltree	   *res;
 
-	end = start + len;
-
 	if (start < 0)
-	{
 		start = t->numlevel + start;
-		end = start + len;
-	}
-	if (start < 0)
-	{							/* start > t->numlevel */
-		start = t->numlevel + start;
-		end = start + len;
-	}
 
 	if (len < 0)
 		end = t->numlevel + len;
 	else if (len == 0)
-		end = (fcinfo->nargs == 3) ? start : 0xffff;
+		end = (fcinfo->nargs == 3) ? start : LTREE_MAX_LEVELS;
+	else
+		end = start + len;
 
 	res = inner_subltree(t, start, end);
 

contrib/ltree/sql/ltree.sql

@@ -34,6 +34,7 @@ SELECT subpath('Top.Child1.Child2',0,0);
 SELECT subpath('Top.Child1.Child2',1,0);
 SELECT subpath('Top.Child1.Child2',0);
 SELECT subpath('Top.Child1.Child2',1);
+SELECT subpath('Top.Child1.Child2',-4);  -- error
 
 
 SELECT index('1.2.3.4.5.6','1.2');