Prevent possible compiler warnings.

Back-patch commit a3e8486dffc413506b14eae9dc5d423f6ae8324a into the 9.0
branch.  This is the only warning I'm seeing in the live branches with
gcc 4.4.7 (other than the known "unused variable 'yyg'" flex lossage),
which makes it just annoying enough to want to clean up.

src/backend/utils/adt/dbsize.c

@@ -609,6 +609,9 @@ pg_relation_filepath(PG_FUNCTION_ARGS)
 		default:
 			/* no storage, return NULL */
 			rnode.relNode = InvalidOid;
+			/* some compilers generate warnings without these next two lines */
+			rnode.dbNode = InvalidOid;
+			rnode.spcNode = InvalidOid;
 			break;
 	}