Crash happens when testing GL_PIXEL_PACK_BUFFER

OS: Ubuntu 20.04 Mesa: master(2021/01/20) GPU name: Intel HD Graphics 630(rev 04)

Note: This issue cannot be reproduced on Mesa 20.0.8.

Case Description:

1. Create a GL_TEXTURE_3D texture texture, levels = 2, [w, h, d] = [4, 4, 4], initialize texture data. 2 Create a GL_PIXEL_PACK_BUFFER buffer buffer. 3 Attach texture level = 1, layer = 1 to a fbo.
2. Call glReadPixels to read fbo content to buffer.
3. Expect buffer content is the same with 'texture'(level = 1, layer = 1)

Reproduced steps:

1. Compile attached file using "g++ -o MipLayerAttachment3DPBO MipLayerAttachment3DPBO.cpp -lX11 -lepoxy";
2. Run "./MipLayerAttachment3DPBO", you could get the result.
3. Crash happens, core dump.

MipLayerAttachment3DPBO.cpp