NULL pointer passed to wglCreatePbufferARB() causes crash w/ Mesa

When a pbuffer is created, in some cases a NULL pointer is passed to wglCreatePbufferARB() for the attribute list.
This code path is hit when Mesa, or a Mesa-based OpenGL implementation - notably the VMware accelerated OpenGL - is used. Unfortunately, Mesa doesn't check whether the attribute list is a NULL pointer and crashes.
The WGL_ARB_pbuffer spec doesn't explicitly allow or disallow NULL pointers; however, the safe approach would be to pass a valid pointer to an empty list instead of simply using NULL. The attached patch does just that.