gallium: no renderbuffer, so just exit.

src/mesa/state_tracker/st_framebuffer.c

@@ -176,7 +176,9 @@ st_set_framebuffer_surface(struct st_framebuffer *stfb,
    assert(surfIndex < BUFFER_COUNT);
 
    strb = st_renderbuffer(stfb->Base.Attachment[surfIndex].Renderbuffer);
-   assert(strb);
+
+   /* fail */
+   if (!strb) return;
 
    /* replace the renderbuffer's surface/texture pointers */
    pipe_surface_reference( &strb->surface, surf );