cd: check that a glob match is successful

Globs that match nothing expand to themselves. This causes `cd --clean`
to have this unwanted behavior:

    ~ $ cd --mounts
    ~ $ cd --clean
    cd: unmounting *... still in use; not unmounting

See: http://mywiki.wooledge.org/NullGlob
Merge pull request #13 from vivien/glob

cd: check that a glob match is successful