Segfault in lpad with UInt64 length

On the official linux 1.1.0 build (also behaves the same in a recent 1.3-DEV):

```
julia> lpad("x", UInt64(1), " ")
"x"

julia> lpad("xx", UInt64(1), " ")

signal (11): Segmentation fault
in expression starting at no file:0
Segmentation fault (core dumped)
```

Originally reported on discourse: https://discourse.julialang.org/t/exception-access-violation-in-windows64/24660/2