Ensure that the xdl_free_classifier() call on xdlclassifier_t cf is safe
even if xdl_init_classifier() isn't called. This may occur in the case
where diff is run with --histogram and a call to, say, xdl_prepare_ctx()
fails.
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>