pgsql: Avoid scribbling on original parsetree during DECLARE CURSOR.

Log Message:
-----------
Avoid scribbling on original parsetree during DECLARE CURSOR. This
prevents problems when the DECLARE is in a portal and is executed
repeatedly, as is possible in v3 protocol. Per analysis by Oliver
Jowett, though I didn't use his patch exactly.

Modified Files:
--------------
pgsql/src/backend/commands:
portalcmds.c (r1.36 -> r1.37)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/portalcmds.c.diff?r1=1.36&r2=1.37)