Fix flipped DTLS checks.

09bd58d1f1 flipped a condition. Doing that
memset in the DTLS case breaks retransmits across a CCS and fails to memset in
the TLS case.

Strangely, it didn't break any tests, but I think that's a function of us
lacking renego tests. The sequence number doesn't seem to be used in the
initial handshake for TLS, so it stayed at zero. After a renego, that codepath
is relevant.

Change-Id: I369a524021857a82e181af7798c7a10fe6279550
Reviewed-on: https://boringssl-review.googlesource.com/1601
Reviewed-by: Adam Langley <agl@google.com>

ssl/t1_enc.c

@@ -495,9 +495,8 @@ int tls1_change_cipher_state(SSL *s, int which)
 	unsigned key_len, iv_len, mac_secret_len;
 	const unsigned char *key_data;
 
-	/* Reset sequence number to zero.
-	 * TODO(davidben): Is this redundant with dtls1_reset_seq_numbers? */
-	if (SSL_IS_DTLS(s))
+	/* Reset sequence number to zero. */
+	if (!SSL_IS_DTLS(s))
 		memset(is_read ? s->s3->read_sequence : s->s3->write_sequence, 0, 8);
 
 	/* key_arg is used for SSLv2. We don't need it for TLS. */