(partially) Fix Bug#55227 Fix compiler warnings in innodb with gcc 4.6

Fix compiler warning:
row/row0purge.c: In function 'row_purge_step':
row/row0purge.c:660:9: error: variable 'err' set but not used [-Werror=unused-but-set-variable]

(row_purge() always returns DB_SUCCESS)

storage/innobase/row/row0purge.c

@@ -667,7 +667,7 @@ row_purge_step(
 
 	err = row_purge(node, thr);
 
-	ut_ad(err == DB_SUCCESS);
+	ut_a(err == DB_SUCCESS);
 
 	return(thr);
 }