evergreeng: protect against null constant buffers

Should do better than this and actually unbind the buffer, but haven't
yet gotten it to work.

src/gallium/drivers/r600/evergreen_state.c

@@ -826,6 +826,13 @@ static void evergreen_set_constant_buffer(struct pipe_context *ctx, uint shader,
 	struct r600_pipe_context *rctx = (struct r600_pipe_context *)ctx;
 	struct r600_resource *rbuffer = (struct r600_resource*)buffer;
 
+	/* Note that the state tracker can unbind constant buffers by
+	 * passing NULL here.
+	 */
+	if (buffer == NULL) {
+		return;
+	}
+
 	switch (shader) {
 	case PIPE_SHADER_VERTEX:
 		rctx->vs_const_buffer.nregs = 0;