app: avoid flushing bufferless projections

Don't needlessly flush projections whose buffer hasn't been
allocated yet.  This can happen when opening an image, in which
case the image is flushed before its projection has a buffer.

app/core/gimpprojection.c

@@ -785,6 +785,9 @@ static void
 gimp_projection_flush_whenever (GimpProjection *proj,
                                 gboolean        now)
 {
+  if (! proj->priv->buffer)
+    return;
+
   if (proj->priv->update_region)
     {
       if (now)  /* Synchronous */