Bug 1478 - Cannot generate or record after starting and stopping Scrub Mode with no audio present

Stream time from the scrubbing audio (which had not started) was coming out as a large negative value.  That in turn led to an out of range selection starting and ending at that value.  Negative stream times now treated as zero when stopping audio.  This does mean that the (fringe) case of stop and set cursor with audio playing to the left of zero, if you are able to do that, will likely set the cursor at zero rather than at the negative time end point.  I think that is probably OK, even good.

src/Menus.cpp

@@ -2397,10 +2397,12 @@ bool AudacityProject::DoPlayStopSelect(bool click, bool shift)
          }
          selection.setTimes(t0, t1);
       }
-      else if (click)
+      else if (click){
+         // avoid a point at negative time.
+         time = wxMax( time, 0 );
          // Set a point selection, as if by a click at the play head
          selection.setTimes(time, time);
-      else
+      } else
          // How stop and set cursor always worked
          // -- change t0, collapsing to point only if t1 was greater
          selection.setT0(time, false);