xwayland/xwm: Avoid zero-size allocaiton

Zero-sized allocations have glibc-specific behavior, so avoid those.

xwayland/xwm.c

@@ -290,7 +290,9 @@ static void xwm_set_net_client_list(struct wlr_xwm *xwm) {
 		}
 	}
 
-	xcb_window_t *windows = malloc(sizeof(xcb_window_t) * mapped_surfaces);
+	xcb_window_t *windows = NULL;
+	if (mapped_surfaces > 0) {
+		xcb_window_t *windows = malloc(sizeof(*windows) * mapped_surfaces);
 		if (!windows) {
 			return;
 		}
@@ -301,6 +303,7 @@ static void xwm_set_net_client_list(struct wlr_xwm *xwm) {
 				windows[index++] = surface->window_id;
 			}
 		}
+	}
 
 	xcb_change_property(xwm->xcb_conn, XCB_PROP_MODE_REPLACE,
 			xwm->screen->root, xwm->atoms[NET_CLIENT_LIST],