Bug fix for cache lookup failure for statistic_ext type

Hackers,

You can easily get a cache lookup failure by changing the regression tests as included in this small patch. The failure looks thus:

+COMMENT ON STATISTICS ab1_a_b_stats IS 'new comment';
+CREATE ROLE temp_role;
+SET SESSION AUTHORIZATION temp_role;
+COMMENT ON STATISTICS ab1_a_b_stats IS 'changed comment';
+ERROR: cache lookup failed for type 27447
+DROP STATISTICS ab1_a_b_stats;
+ERROR: cache lookup failed for type 27447
+ALTER STATISTICS ab1_a_b_stats RENAME TO ab1_a_b_stats_new;
+ERROR: must be owner of statistics object ab1_a_b_stats
+RESET SESSION AUTHORIZATION;
+DROP ROLE temp_role;

I believe this case simply has not had any test coverage, as I don't see any way the current code would ever work. It treats the Oid of the statistics object as a type, which it is not.