pgsql: Fix minor memory leak introduced in recent SQL-functions hacking:

Log Message:
-----------
Fix minor memory leak introduced in recent SQL-functions hacking: the
DestReceiver created during postquel_start needs to be destroyed during
postquel_end.  In a moment of brain fade I had assumed this would be taken
care of by FreeQueryDesc, but it's not (and shouldn't be).

Modified Files:
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    pgsql/src/backend/executor:
        functions.c (r1.128 -> r1.129)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/functions.c?r1=1.128&r2=1.129)