Maybe a bug about built-in string to cstring converter

[Ward]

    proc test(x: cstring) = echo cast[int](x)
    var str: string = nil
    test(nil) # output 0 -> correct
    test(str) # output 8 ??
    

I think this should be a bug.

I use "Nim Compiler Version 0.15.2 (2016-10-23) [Windows: i386]"