ui-shared: don't print path crumbs without a repo

ui-shared: don't print path crumbs without a repo

cgit_print_path_crumbs() can call repolink() which assumes that ctx.repo
is non-null.  Currently we don't have any commands that set want_vpath
without also setting want_repo so it shouldn't be possible to fail this
test, but the check in cgit.c is in the wrong order so it is possible to
specify a query string like "?p=log&path=foo/bar" to end up here without
a valid repository.

This was found by American fuzzy lop [0].

[0] http://lcamtuf.coredump.cx/afl/

Signed-off-by: John Keeping <john@keeping.me.uk>