softpipe: Prevent NULL ptr derreference on takedown.

src/gallium/drivers/softpipe/sp_state_fs.c

@@ -154,8 +154,8 @@ softpipe_set_constant_buffer(struct pipe_context *pipe,
    /* note: reference counting */
    pipe_buffer_reference(ws,
 			 &softpipe->constants[shader].buffer,
-			 buf->buffer);
-   softpipe->constants[shader].size = buf->size;
+			 buf ? buf->buffer : NULL);
+   softpipe->constants[shader].size = buf ? buf->size : 0;
 
    softpipe->dirty |= SP_NEW_CONSTANTS;
 }