plproxy - plproxy: Fix argument value NULL check.

Log Message:
-----------
Fix argument value NULL check.

Incoming NULL value could cause crash in function containing SELECT
with different argument order. Due to thinko, NULL check was done
with query arg index, instead of function arg index.

Reported by João Matos

Modified Files:
--------------
plproxy/src:
execute.c (r1.16 -> r1.17)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plproxy/plproxy/src/execute.c?r1=1.16&r2=1.17)
plproxy/sql:
plproxy_select.sql (r1.3 -> r1.4)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plproxy/plproxy/sql/plproxy_select.sql?r1=1.3&r2=1.4)
plproxy/expected:
plproxy_select.out (r1.4 -> r1.5)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/plproxy/plproxy/expected/plproxy_select.out?r1=1.4&r2=1.5)