fix oversight converting buf_id to Buffer

Attached patch fixes oversights converting buf_id to Buffer in
PrintBufferDescs() and InvalidateBuffer(). Especially for the latter,
the reason we haven't seen any reports of the issue might be that it
needs certain concurrent conditions to be true.

Along the line, it also changes all direct maths against buf_id to use
BufferDescriptorGetBuffer() instead.

Regards,
Qingqing