worktree: don't segfault with an absolute pathspec without a work tree

worktree: don't segfault with an absolute pathspec without a work tree

If a command is run with an absolute path as a pathspec inside a bare
repository, e.g. "rev-list HEAD -- /home", the code tried to run strlen()
on NULL, which is the result of get_git_work_tree(), and segfaulted.  It
should just fail instead.

Currently the function returns NULL even inside .git/ in a repository
with a work tree, but that is a separate issue.

Signed-off-by: Junio C Hamano <gitster@pobox.com>