pgsql: Backpatch fix for buffer overrun in parsing refcursor parameters

Log Message:
-----------
Backpatch fix for buffer overrun in parsing refcursor parameters to
REL7_3_STABLE.

Tags:
----
REL7_3_STABLE

Modified Files:
--------------
pgsql/src/pl/plpgsql/src:
gram.y (r1.39 -> r1.39.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.39&r2=1.39.2.1)