app: fix CRITICAL when using "show all" by default

When using "show all" by default, gimp_display_flush() can be
called during GimpDisplayShell construction, before the newly-
constructed shell is assigned to the display.  Use an ugly hack to
just ignore the flush when this happens.

(cherry picked from commit 0a06294af7)

app/display/gimpdisplay.c

@@ -870,6 +870,11 @@ gimp_display_flush (GimpDisplay *display)
 {
   g_return_if_fail (GIMP_IS_DISPLAY (display));
 
+  /* FIXME: we can end up being called during shell construction if "show all"
+   * is enabled by default, in which case the shell's display pointer is still
+   * NULL
+   */
+  if (gimp_display_get_shell (display))
     gimp_display_flush_whenever (display, FALSE);
 }