config --get --path: check for unset $HOME

If $HOME is unset (as in some automated build situations),
currently

git config --path path.home "~"
git config --path --get path.home

segfaults.  Error out with

Failed to expand user dir in: '~/'

instead.

Reported-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>