Initialized `_root` properly

Fix #4204

`_root` is uninitialized for non-root nodes. And it will cause `module_node::mount` fail because it uses `root()`. Once the bug is triggered, signal 11 is received but Magisk catch all signals and therefore stuck forever.

native/jni/core/module.cpp

@@ -224,7 +224,7 @@ protected:
     map_type children;
 
     // Root node lookup cache
-    root_node *_root;
+    root_node *_root = nullptr;
 };
 
 class root_node : public dir_node {