backend: Do not attempt DRM on X11/WL failure

This can really mess with the session if logind is not being used,
and it's going to always fail anyway.

backend/backend.c

@@ -297,22 +297,26 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 			getenv("WAYLAND_SOCKET")) {
 		struct wlr_backend *wl_backend = attempt_wl_backend(display,
 			create_renderer_func);
-		if (wl_backend) {
+		if (!wl_backend) {
+			goto error;
+		}
+
 		wlr_multi_backend_add(backend, wl_backend);
 		return backend;
 	}
-	}
 
 #if WLR_HAS_X11_BACKEND
 	const char *x11_display = getenv("DISPLAY");
 	if (x11_display) {
 		struct wlr_backend *x11_backend =
 			attempt_x11_backend(display, x11_display, create_renderer_func);
-		if (x11_backend) {
+		if (!x11_backend) {
+			goto error;
+		}
+
 		wlr_multi_backend_add(backend, x11_backend);
 		return backend;
 	}
-	}
 #endif
 
 	// Attempt DRM+libinput
@@ -344,4 +348,8 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 	}
 
 	return backend;
+
+error:
+	wlr_backend_destroy(backend);
+	return NULL;
 }