drm/panthor: Fix potential invalid pointer deref in group_process_tiler_oom()

drm/panthor: Fix potential invalid pointer deref in group_process_tiler_oom()

If heaps is an ERR_PTR(), panthor_heap_pool_put() will deref an invalid
pointer. Make sure we set it to NULL in that case.

Fixes: de8548813824 ("drm/panthor: Add the scheduler logical block")
Reported-by: sashiko-bot@kernel.org
Closes: https://sashiko.dev/#/patchset/20260625-panthor-signal-from-irq-v5-0-8836a74e0ef9@collabora.com?part=2
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260625-panthor-misc-fixes-v1-4-b67ed973fea6@collabora.com