pgsql: Fix unwarranted assumption that a cached rowtype would stick

Log Message:
-----------
Fix unwarranted assumption that a cached rowtype would stick around
for the lifespan of the CreateStmt.  Per buildfarm member jaguar.

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_utilcmd.c (r2.37 -> r2.38)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_utilcmd.c?r1=2.37&r2=2.38)