Fix error with PG17 palloc0fast compilation

[uhayat]

In PG17 palloc0fast is removed and palloc0 is instead

[shinyaaa]

I think that change is correct.
However, in this project, the condition by PG_VERSION_NUM may be unnecessary, as the branches are separated by PG version.
(I don't know how to do this, as the PG17 branch has not yet been created.)

[jrgemignani]

@uhayat I am okay with merging a change where palloc0fast is replaced by palloc0. But, we don't use version flags as stated by @shinyaaa above, so I won't approve that.

[uhayat]

@jrgemignani understood. I updated the commit and removed the version check.

[uhayat]

Anybody available for review ?

[jrgemignani]

@uhayat See #2111 (comment)