Fixes #6626: Throw if Lv2 object CTORs fail (#6951)

On plugin instantiation failure, `Lv2Proc::m_valid` was being set to false. However, `Lv2Proc::run` did not evaluate `m_valid` and still called `lilv_instance_run`, which caused undefined behavior, including crashes.

This bug fixes this by not even create such zombie classes, and instead `throw`s right away. The throws are caught in `lmms_plugin_main`, as suggested in the PR discussion and as the VST3 approach.

include/Lv2ControlBase.h

@@ -102,9 +102,6 @@ protected:
 
 	Lv2ControlBase& operator=(const Lv2ControlBase&) = delete;
 
-	//! Must be checked after ctor or reload
-	bool isValid() const { return m_valid; }
-
 	/*
 		overrides
 	*/
@@ -149,7 +146,6 @@ private:
 	//! fulfill LMMS' requirement of having stereo input and output
 	std::vector<std::unique_ptr<Lv2Proc>> m_procs;
 
-	bool m_valid = true;
 	bool m_hasGUI = false;
 	unsigned m_channelsPerProc;
 

include/Lv2Proc.h

@@ -78,8 +78,6 @@ public:
 	~Lv2Proc() override;
 	void reload();
 	void onSampleRateChanged();
-	//! Must be checked after ctor or reload
-	bool isValid() const { return m_valid; }
 
 	/*
 		port access
@@ -173,8 +171,6 @@ protected:
 	void shutdownPlugin();
 
 private:
-	bool m_valid = true;
-
 	const LilvPlugin* m_plugin;
 	LilvInstance* m_instance = nullptr;
 	Lv2Features m_features;