diffcore-break.c: check diff_delta() return value.

This bug caused Darrin Thompson to notice that our deltifier was
half broken and punting on an empty blob.
Signed-off-by: NJunio C Hamano <junio@twinsun.com>

diffcore-break.c

@@ -66,6 +66,8 @@ static int should_break(struct diff_filespec *src,
 	delta = diff_delta(src->data, src->size,
 			   dst->data, dst->size,
 			   &delta_size, 0);
+	if (!delta)
+		return 0; /* error but caught downstream */
 
 	/* Estimate the edit size by interpreting delta. */
 	if (count_delta(delta, delta_size,