[PATCH 2/2] erofs-utils: libzstd: fix undefined behavior shift in setdictsize

[Nithurshen]

Hi Xiang,

I do agree that `pclustersize_max` is always greater than 0 and
`ondisk_extradevs` is at max 65535.

My intent was to look at these functions as independent blocks.
Even though the current upstream callers provide safe inputs, I
wanted to make them robust against any undefined behavior in case
they are reused in the future.

Thanks,
Nithurshen

Regards,
Nithurshen