egl/dri: Don't invoke dri2_dpy->flush if it's NULL.

I'd like to test Mesa OpenGL ES along side with NVIDIA libGL drivers. But
without this change, I get a NULL pointer dereference.

Reviewed-by: Brian Paul <brianp@vmware.com>
diff --git a/src/egl/drivers/dri2/egl_dri2.c b/src/egl/drivers/dri2/egl_dri2.c
index 0ffbc68..ae842d7 100644
--- a/src/egl/drivers/dri2/egl_dri2.c
+++ b/src/egl/drivers/dri2/egl_dri2.c
@@ -976,7 +976,8 @@
    /* FIXME: If EGL allows frontbuffer rendering for window surfaces,
     * we need to copy fake to real here.*/
 
-   (*dri2_dpy->flush->flush)(dri2_surf->dri_drawable);
+   if (dri2_dpy->flush != NULL)
+      dri2_dpy->flush->flush(dri2_surf->dri_drawable);
 
    return EGL_TRUE;
 }