initialize scope owner in default TempoMap constructor

This constructor should not really exist, but it does.

libs/temporal/temporal/tempo.h

@@ -771,7 +771,7 @@ class /*LIBTEMPORAL_API*/ TempoMap : public PBD::StatefulDestructible
 	/* and now on with the rest of the show ... */
 
   public:
-	LIBTEMPORAL_API TempoMap () {}
+	LIBTEMPORAL_API TempoMap () : _scope_owner (nullptr) {}
 	LIBTEMPORAL_API TempoMap (Tempo const& initial_tempo, Meter const& initial_meter);
 	LIBTEMPORAL_API TempoMap (TempoMap const&);
 	LIBTEMPORAL_API TempoMap (XMLNode const&, int version);