plproxy - plproxy: Reset cluster_sql on parser entry.

Log Message:
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Reset cluster_sql on parser entry.

Fixes crash in case CLUSTER 'name'; follows CLUSTER func();

Modified Files:
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plproxy/src:
parser.y (r1.5 -> r1.6)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plproxy/plproxy/src/parser.y.diff?r1=1.5&r2=1.6)