Fix crash in wxGauge with wxGA_PROGRESS under macOS

Hide, instead of destroying, the progress indicator in reset() to
prevent crashes if SetValue() is called later.

Closes https://github.com/wxWidgets/wxWidgets/pull/1137

src/osx/cocoa/appprogress.mm

@@ -65,7 +65,9 @@
 
 - (void)reset
 {
-    [m_dockTile setContentView:nil];
+    [m_progIndicator setHidden:YES];
+
+    [m_dockTile display];
 }
 
 @end