svga: Add an assert to catch reentrancy.

src/gallium/drivers/svga/svga_swtnl_backend.c

@@ -90,6 +90,7 @@ svga_vbuf_render_allocate_vertices( struct vbuf_render *render,
                                              svga_render->vbuf_size);
       if(!svga_render->vbuf) {
          svga_context_flush(svga, NULL);
+         assert(!svga_render->vbuf);
          svga_render->vbuf = pipe_buffer_create(screen,
                                                 PIPE_BIND_VERTEX_BUFFER,
                                                 svga_render->vbuf_size);