Fix uninitialized value

as reported when compiling with optimizations (-O2).

src/plugins/node_features/knl_generic/node_features_knl_generic.c

@@ -1423,6 +1423,7 @@ extern int node_features_p_node_set(char *active_features)
 				key = "Memory";
 				break;
 			default:
+				key = NULL;
 				break;
 			}
 		else if (strstr(active_features, "hybrid"))