Assert(IsTransactionState()) in RelationIdGetRelation().

Commit 42c80c696e9c8323841180029cc62741c21bd356 added an
Assert(IsTransactionState()) in SearchCatCache(), to catch
any code that thought it could do a catcache lookup outside
transactions.  Extend the same idea to relcache lookups.

src/backend/utils/cache/relcache.c

@@ -1578,6 +1578,9 @@ RelationIdGetRelation(Oid relationId)
 {
 	Relation	rd;
 
+	/* Make sure we're in an xact, even if this ends up being a cache hit */
+	Assert(IsTransactionState());
+
 	/*
 	 * first try to find reldesc in the cache
 	 */