0x0 GLXPbuffers are underspecified

Mesa has never supported this, and I believe NVIDIA doesn't either, and it's a fairly goofy thing to do in any case. However the spec doesn't say what error to raise to indicate failure here. Mesa happens to throw BadValue in its real-GLX paths, and will simply return 0 for fake-GLX.

A similar observation applies to EGLPbuffers, but there are probably many more EGL implementations than GLX. In any case I'm happy to simply codify whatever error is most common, and ideally update the specs to indicate that 0x0 might not be supported.