Fix out-of-bound read in gtsvector_picksplit()

Fix out-of-bound read in gtsvector_picksplit()

This could lead to an imprecise choice when splitting an index page of a
GiST index on a tsvector, deciding which entries should remain on the
old page and which entries should move to a new page.

This is wrong since tsearch2 has been moved into core with commit
140d4ebcb46e, so backpatch all the way down.  This error has been
spotted by valgrind.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/17950-6c80a8d2b94ec695@postgresql.org
Backpatch-through: 11