pgsql: Prevent memory leaks in our various bison parsers when an error

Log Message:
-----------
Prevent memory leaks in our various bison parsers when an error occurs
during parsing.  Formerly the parser's stack was allocated with malloc
and so wouldn't be reclaimed; this patch makes it use palloc instead,
so that flushing the current context will reclaim the memory.  Per
Marko Kreen.

Modified Files:
--------------
    pgsql/contrib/cube:
        cubeparse.y (r1.17 -> r1.18)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/cube/cubeparse.y?r1=1.17&r2=1.18)
    pgsql/contrib/seg:
        segparse.y (r1.17 -> r1.18)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/seg/segparse.y?r1=1.17&r2=1.18)
    pgsql/src/backend/bootstrap:
        bootparse.y (r1.93 -> r1.94)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/bootstrap/bootparse.y?r1=1.93&r2=1.94)
    pgsql/src/backend/parser:
        gram.y (r2.621 -> r2.622)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.621&r2=2.622)
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.113 -> r1.114)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.113&r2=1.114)