[BUG] git-check-ignore: Segmentation fault

Hi there,

The new git-check-ignore command seg faults when
    (1) it is called with single dot path name at $GIT_DIR level  _AND_
    (2) and .gitignore has at least one directory pattern.

Git version: 1.8.2.rc0.16.g20a599e

Reproduce the bug:
    $ git --version
    git version 1.8.2.rc0.16.g20a599e
    $ mkdir test
    $ cd test
    $ git init
    $ git check-ignore .  # All good, no errors here
    $ echo "dirpattern/" > .gitignore
    $ git check-ignore .
    Segmentation fault (core dumped)

The segmentation fault is actually caused by hash_name(const char *name, int namelen) function in name-hash.c when the 'name' argument is an empty stringi and namelen is 0.

The empty string comes from a call to the prefix_path(prefix, len, path) function in setup.c. In this instance arguments 'prefix' is NULL, 'len' is 0 and 'path' is "." .

Cheers, Zoltan