Add a method to clear ImportStatus arrays

This is in preparation to fix a
 "SessionHandleRef exists across session deletion"
error.

libs/ardour/ardour/import_status.h

@@ -34,6 +34,15 @@ namespace ARDOUR {
 
 class LIBARDOUR_API ImportStatus : public InterThreadInfo {
 public:
+	virtual ~ImportStatus() {
+		clear ();
+	}
+
+	virtual void clear () {
+		sources.clear ();
+		paths.clear ();
+	}
+
 	std::string doing_what;
 
 	/* control info */