drm/xe/pt: Fix NULL pointer dereference in xe_pt_zap_ptes_entry()

drm/xe/pt: Fix NULL pointer dereference in xe_pt_zap_ptes_entry()

The page-table walk framework may pass a NULL *child pointer for
unpopulated entries. xe_pt_zap_ptes_entry() called container_of(*child)
before checking for NULL, then dereferenced the result, causing a crash.

Move the container_of() call after a NULL guard, so the function returns
early instead of proceeding with an invalid pointer. XE_WARN_ON is kept
to help root cause the issue, but we now bail instead of crashing the
driver.

v2: Comment that triggering XE_WARN_ON is unexpected behavior (Matt Brost)

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20260616081756.286918-1-francois.dugast@intel.com
Signed-off-by: Francois Dugast <francois.dugast@intel.com>