test get_git_work_tree() return value for NULL

test get_git_work_tree() return value for NULL

If we are in a git directory, get_git_work_tree() can return NULL.
While trying to determine whether or not the given paths are outside
the work tree, the following command would read from it anyways and
trigger a segmentation fault.

 git diff / /

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>