builtin/blame.c: Fix a "Using plain integer as NULL pointer" warning

Plain gcc may not but sparse catches and complains about this sort of
stuff.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -93,7 +93,7 @@ static int diff_hunks(mmfile_t *file_a, mmfile_t *file_b, long ctxlen,
 {
 	xpparam_t xpp = {0};
 	xdemitconf_t xecfg = {0};
-	xdemitcb_t ecb = {0};
+	xdemitcb_t ecb = {NULL};
 
 	xpp.flags = xdl_opts;
 	xecfg.ctxlen = ctxlen;