(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:687:9: error: variable 'err' set but not used [-Werror=unused-but-set-variable]

storage/innodb_plugin/row/row0purge.c

@@ -684,7 +684,9 @@ row_purge_step(
 	que_thr_t*	thr)	/*!< in: query thread */
 {
 	purge_node_t*	node;
+#ifdef UNIV_DEBUG
 	ulint		err;
+#endif /* UNIV_DEBUG */
 
 	ut_ad(thr);
 
@@ -692,7 +694,10 @@ row_purge_step(
 
 	ut_ad(que_node_get_type(node) == QUE_NODE_PURGE);
 
-	err = row_purge(node, thr);
+#ifdef UNIV_DEBUG
+	err =
+#endif /* UNIV_DEBUG */
+	row_purge(node, thr);
 
 	ut_ad(err == DB_SUCCESS);