Refactor/Reformat tls_pre_decrypt

Refactor/Reformat tls_pre_decrypt

- Extract data packet handling to its own function
- Replace two instances of
          if (x) { code }
  with
          if (!x) return; code

- Remove extra curly braces that were used for pre C99 code style
  to be able to declare variables in the middle of a block

This patch is easier to review with "ignore white space" as the
diff is then a lot smaller in that case and the changes more obvious.

Patch V2: Fix function name spelling, cleanup goto code in the new
          handle_data_channel_packet function

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200811105541.2543-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20707.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>