diff.c: free "buf" in diff_words_flush()

Amend the freeing logic added in e6e045f803 (diff.c: buffer all
output if asked to, 2017-06-29) to free the containing "buf" in
addition to its members.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2150,6 +2150,7 @@ static void diff_words_flush(struct emit_callback *ecbdata)
 
 		for (i = 0; i < wol->nr; i++)
 			free((void *)wol->buf[i].line);
+		free(wol->buf);
 
 		wol->nr = 0;
 	}