Lv2Proc: Fix shutdown routine (#6944)

include/Lv2Options.h

@@ -84,6 +84,8 @@ public:
 		return m_options.data();
 	}
 
+	void clear();
+
 private:
 	//! Initialize an option internally
 	void initOption(LV2_URID key,

include/Lv2Proc.h

@@ -175,7 +175,7 @@ private:
 	bool m_valid = true;
 
 	const LilvPlugin* m_plugin;
-	LilvInstance* m_instance;
+	LilvInstance* m_instance = nullptr;
 	Lv2Features m_features;
 
 	// options