[PATCH] iommu/riscv: check pte null pointer before use

[Markus Elfring]

…
> This commit just check the pointer before using it to avoid the bug.
…

See also:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.16#n94
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?h=v6.16#n145

Regards,
Markus