pgsql: Fix thinko in tok_is_keyword(): it was looking at the wrong union

Log Message:
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Fix thinko in tok_is_keyword(): it was looking at the wrong union variant
of YYSTYPE, and hence returning the wrong answer for cases where a plpgsql
"unreserved keyword" really does conflict with a variable name. Obviously
I didn't test this enough :-(. Per bug #5524 from Peter Gagarinov.

Modified Files:
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pgsql/src/pl/plpgsql/src:
gram.y (r1.142 -> r1.143)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.142&r2=1.143)
pgsql/src/test/regress/expected:
plpgsql.out (r1.84 -> r1.85)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/plpgsql.out?r1=1.84&r2=1.85)
pgsql/src/test/regress/sql:
plpgsql.sql (r1.71 -> r1.72)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/plpgsql.sql?r1=1.71&r2=1.72)