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
Don't you have to handle the `m == nothing` case?

See also my [comment](https://github.com/JuliaLang/julia/pull/8838#discussion_r21249901).  It's not clear to me why the original code is incorrect, since `m.offset` is the offset of an ASCII character (a single code unit).