compositor: Don't free an uninitialised pointer.

When backend_init returns NULL, we goto out_signals, which wants to
free(modules), but in this particular code path, modules hasn't been
initialised leading to a "Double-free or corruption" error message.
Initialising modules to NULL makes the free a no-op in this scenario.

Signed-off-by: Ondřej Majerech <majerech.o@gmail.com>
Reviewed-by: Ryo Munakata <ryomnktml@gmail.com>

src/compositor.c

@@ -4335,7 +4335,8 @@ int main(int argc, char *argv[])
 	int i, fd;
 	char *backend = NULL;
 	char *shell = NULL;
-	char *modules, *option_modules = NULL;
+	char *modules = NULL;
+	char *option_modules = NULL;
 	char *log = NULL;
 	char *server_socket = NULL, *end;
 	int32_t idle_time = 300;