pgsql: Fix core dump in QTNodeCompare when tsquery_cmp() is applied to

Log Message:
-----------
Fix core dump in QTNodeCompare when tsquery_cmp() is applied to two empty
tsqueries. CompareTSQ has to have a guard for the case rather than blindly
applying QTNodeCompare to random data past the end of the datums. Also,
change QTNodeCompare to be a little less trusting: use an actual test rather
than just Assert'ing that the input is sane. Problem encountered while
investigating another issue (I saw a core dump in autoanalyze on a table
containing multiple empty tsquery values).

Back-patch to all branches with tsquery support.

In HEAD, also fix some bizarre (though not outright wrong) coding in
tsq_mcontains().

Modified Files:
--------------
pgsql/src/backend/utils/adt:
tsquery_op.c (r1.8 -> r1.9)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery_op.c?r1=1.8&r2=1.9)
tsquery_util.c (r1.13 -> r1.14)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/tsquery_util.c?r1=1.13&r2=1.14)