Fix TLS 1.3 seal logic.

Check against the write encryption state, not the read state.

Change-Id: Ib3d8e02800e37bd089ef02c67a0b7e5dc009b1a5
Reviewed-on: https://boringssl-review.googlesource.com/8330
Reviewed-by: Nick Harper <nharper@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>

ssl/tls_record.c

@@ -360,7 +360,7 @@ int tls_seal_record(SSL *ssl, uint8_t *out, size_t *out_len, size_t max_out,
   /* TLS 1.3 hides the actual record type inside the encrypted data. */
   if (ssl->s3->have_version &&
       ssl3_protocol_version(ssl) >= TLS1_3_VERSION &&
-      ssl->s3->aead_read_ctx != NULL) {
+      ssl->s3->aead_write_ctx != NULL) {
     size_t padding = SSL3_RT_HEADER_LENGTH + 1;
 
     if (in_len > in_len + padding || max_out < in_len + padding) {