vmalloc: fix NULL pointer dereference in is_vm_area_hugepages()

vmalloc: fix NULL pointer dereference in is_vm_area_hugepages()

find_vm_area() can return NULL if the given address is not a valid vmalloc
area.  Check the return value before dereferencing it to avoid a kernel
crash.

Link: https://lore.kernel.org/20260529014130.671291-1-hui.zhu@linux.dev
Fixes: 121e6f3258fe ("mm/vmalloc: hugepage vmalloc mappings")
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>