CVE-2026-98270: drm/amdgpu: check ras and obj before dereference

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: check ras and obj before dereference

nbiov79handlerascontrollerintrnobifring() dereferences ras and obj without checking either for NULL. Both amdgpurasgetcontext() and amdgpurasfindobj() can return NULL, e.g. during the window between adev->nbio.ras being set (early in amdgpurasinit(), by design, to enable the fatal-error interrupt as soon as possible) and the PCIEBIF ras object actually being created in RAS lateinit. Any interrupt in that window crashes in hard-IRQ context.

This is analogous to commit d190b459b2a4 ("drm/amdgpu: the warning dereferencing obj for nbiov74"), which fixed the same issue in the nbiov74 handler.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

(cherry picked from commit c7071767a50a32ed727cf800ac84372429e3b4b3)