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>

src/mesa/drivers/dri/common/dri_util.c

@@ -645,6 +645,8 @@ driCreateNewDrawable(__DRIscreen *screen,
 {
     __DRIdrawable *pdraw;
 
+    assert(data != NULL);
+
     pdraw = malloc(sizeof *pdraw);
     if (!pdraw)
 	return NULL;
@@ -674,6 +676,16 @@ driCreateNewDrawable(__DRIscreen *screen,
 static void
 driDestroyDrawable(__DRIdrawable *pdp)
 {
+    /*
+     * The loader's data structures are going away, even if pdp itself stays
+     * around for the time being because it is currently bound. This happens
+     * when a currently bound GLX pixmap is destroyed.
+     *
+     * Clear out the pointer back into the loader's data structures to avoid
+     * accessing an outdated pointer.
+     */
+    pdp->loaderPrivate = NULL;
+
     dri_put_drawable(pdp);
 }