More of bug 1338: avoid Mac hang with modal invocation of plugin...

... This required a separate set of changes and testing for the VST version of
SPAN, another for the AU version.

see commit c8e570797f

src/effects/audiounits/AUControl.mm

@@ -107,12 +107,18 @@ AUControl::AUControl()
 }
 
 AUControl::~AUControl()
+{
+   Close();
+}
+
+void AUControl::Close()
 {
 #if !defined(_LP64)
 
    if (mInstance)
    {
       AudioComponentInstanceDispose(mInstance);
+      mInstance = nullptr;
    }
 
 #endif
@@ -125,11 +131,13 @@ AUControl::~AUControl()
                       object:mView];
 
       [mView release];
+      mView = nullptr;
    }
 
    if (mAUView)
    {
       [mAUView release];
+      mAUView = nullptr;
    }
 }