Avoid unsafe intermediate slice construction in varlena functions

[SergeiPatiakin]

unsafe __IncompleteArrayField<T>::as_slice calls unsafe ::core::slice::from_raw_parts, which is UB if the slice memory range is invalid.

But the length we pass in the varlena_4b* cases is varsize_1b(ptr) which is a garbage value
potentially larger than the real length. Therefore the slice memory range can indeed be invalid.

I don't think this is a practical concern today because we immediately do .as_ptr on
the intermediate slice, but still looks like technically UB.

[0xPoe]

Although it doesn’t cause any issues now, it’s reasonable to fix it.