xdiff/xprepare: initialise xdlclassifier_t cf in xdl_prepare_env()

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>

xdiff/xprepare.c

@@ -239,6 +239,8 @@ int xdl_prepare_env(mmfile_t *mf1, mmfile_t *mf2, xpparam_t const *xpp,
 	long enl1, enl2, sample;
 	xdlclassifier_t cf;
 
+	memset(&cf, 0, sizeof(cf));
+
 	/*
 	 * For histogram diff, we can afford a smaller sample size and
 	 * thus a poorer estimate of the number of lines, as the hash