compositor: Set WAYLAND_DISPLAY only after creating backend

The wayland backend needs to connect to the original WAYLAND_DISPLAY
before we overwrite it.

https://bugs.freedesktop.org/show_bug.cgi?id=53064

src/compositor.c

@@ -3535,8 +3535,6 @@ int main(int argc, char *argv[])
 
 	config_file = config_file_path("weston.ini");
 	parse_config_file(config_file, cs, ARRAY_LENGTH(cs), shell);
-	if (socket_name)
-		setenv("WAYLAND_DISPLAY", socket_name, 1);
 
 	backend_init = load_module(backend, "backend_init", &backend_module);
 	if (!backend_init)
@@ -3576,6 +3574,9 @@ int main(int argc, char *argv[])
 		goto out;
 	}
 
+	if (socket_name)
+		setenv("WAYLAND_DISPLAY", socket_name, 1);
+
 	if (!shell)
 		shell = "desktop-shell.so";
 	module_init = load_module(shell, "shell_init", &shell_module);