pgsql: Fix tsquerysel() to not fail on an empty TSQuery.

Log Message:
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Fix tsquerysel() to not fail on an empty TSQuery. Per report from
Tatsuo Ishii.

Modified Files:
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pgsql/src/backend/tsearch:
ts_selfuncs.c (r1.2 -> r1.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tsearch/ts_selfuncs.c?r1=1.2&r2=1.3)