In attach() check for pDraw being NULL, and also print an ErrorF message,
because we eventually want to track down why this is occuring.

It's unclear how this occurs, but as I noted in the 1.4 branch, I believe that
the DrawablePtr/struct _Drawable -> id is the member being accessed that causes
KERN_PROTECTION_FAILURE at 0x0000000000000004

This passes my tests using: env LIBGL_ALWAYS_INDIRECT=1 ./sometest.

I fixed a warning: caused by initializing the screen->base.visuals with the
configs.  It is a ** not a *.  It seems that some other part of GLX will
initialize this for us.
(cherry picked from commit 17f6a261)