[AudioUnit] Only create the GCHandle to self when we need it. (#13397)

src/AudioUnit/AudioUnit.cs

@@ -312,7 +312,6 @@ namespace AudioUnit
 		internal AudioUnit (IntPtr handle, bool owns)
 			: base (handle, owns)
 		{
-			gcHandle = GCHandle.Alloc(this);
 		}
 
 		static IntPtr Create (AudioComponent component)
@@ -601,6 +600,9 @@ namespace AudioUnit
 
 			renderer [audioUnitElement] = renderDelegate;
 
+			if (!gcHandle.IsAllocated)
+				gcHandle = GCHandle.Alloc (this);
+
 			var cb = new AURenderCallbackStruct ();
 			cb.Proc = CreateRenderCallback;
 			cb.ProcRefCon = GCHandle.ToIntPtr (gcHandle);