Fix uninitialised variables

When importing an AUP project, selLow and selHigh may not be defined
in the project. If not defined, this causes an error on initialising
Nyquist. See: https://forum.audacityteam.org/viewtopic.php?f=48&t=118940

src/import/ImportAUP.cpp

@@ -171,8 +171,8 @@ private:
       field(sel0, double);
       field(sel1, double);
 #ifdef EXPERIMENTAL_SPECTRAL_EDITING
-      field(selLow, double);
-      field(selHigh, double);
+      field(selLow, double) = SelectedRegion::UndefinedFrequency;
+      field(selHigh, double) = SelectedRegion::UndefinedFrequency;
 #endif
       field(rate, double);
       field(snapto, bool);