Segfault in lpad with UInt64 length

[c42f]

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

[c42f]

The logic of lpad/rpad is wrong for unsigned conversions, but this just further exposes the following:

julia> repeat('x', UInt64(0)-1)

signal (11): Segmentation fault
in expression starting at no file:0
Segmentation fault (core dumped)