glx/dri3: handle NULL pointers in loader-to-DRI3 drawable conversion

glx/dri3: handle NULL pointers in loader-to-DRI3 drawable conversion

With a subsequent patch, we might see NULL loaderPrivates, e.g. when
a DRIdrawable is flushed whose corresponding GLXDRIdrawable was destroyed.
This resulted in a crash, since the loader vs. DRI3 drawable structures
have a non-zero offset.

Fixes glx-visuals-{depth,stencil} -pixmap

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