mm/kasan: fix null pointer dereference warning in qlink_to_cache()

mm/kasan: fix null pointer dereference warning in qlink_to_cache()

virt_to_slab() declared in slab.h can return NULL if the address does not
belong to a slab.  This case is not handled in qlink_to_cache() in
quarantine.c, which can cause a NULL pointer dereference in
"virt_to_slab(qlink)->slab_cache".  This issue was discovered by fanalyzer
(my gcc version: 12.1.1 20220507)

Link: https://lkml.kernel.org/r/20220626170355.198913-1-gautammenghani201@gmail.com
Signed-off-by: Gautam Menghani <gautammenghani201@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>