dri/common: clear the loaderPrivate pointer in driDestroyDrawable

dri/common: clear the loaderPrivate pointer in driDestroyDrawable

The GLX specification says about glXDestroyPixmap:

    "The storage for the GLX pixmap will be freed when it is not current
     to any client."

We're not really following this language to the letter: some of the storage
is freed immediately (in particular, the dri3_drawable, which contains both
GLXDRIdrawable and loader_dri3_drawable). So we NULL out the pointers to
that freed storage; the previous patches added the corresponding NULL-pointer
checks.

This fixes memory corruption in piglit
./bin/glx-visuals-depth/stencil -pixmap -auto

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>