Finish PR: inference: fix the ptrfree field check

[MasonProtter]

This should address the # TODO item in #57550.

Unconditionally indexing DataTypeFieldDesc(dt::DataType)[fidx] will error when dt has no layout because we define

struct DataTypeFieldDesc
    dt::DataType
    function DataTypeFieldDesc(dt::DataType)
        dt.layout == C_NULL && throw(UndefRefError())
        new(dt)
    end
end

For all the DataTypes I could find, they only had dt.layout == C_NULL if !isconcretetype(dt), so maybe it would be preferable to check if they're !isconcretetype rather than checking if dt.layout == C_NULL, but I figured it'd be safer to just mimic the validation logic in DataTypeFieldDesc instead.

[aviatesk]

Thanks!