Fix path completion when UTF8 char is occurring before match characters.

Small fix to #8838 so completion works again on: `"β $dir_space\\space"`, @tkelman, @ivarne
@stevengj thank you for the pointer to endof, it was wrong to use `length`. But it still seems like it should be `start = nextind(s,endof(s)) - m.offset + 1` to get from `C`.
```julia
julia>s="αβ C:/αβ"
julia>start = match(r" ",reverse(s)).offset
julia>s[nextind(s,endof(s)) - start + 1:end]
"C:/αβ"
```