Update sylpheed to 3.4.3

* 3.4.3 (stable)

    * The bug that wrote the first part of data if the message body in the
      IMAP4 responses didn't end with CR+LF was fixed (#84).
    * The crash when displaying HTML messages was fixed (#215).
    * Win32: the crash when linked with newer MSVCRT was fixed.
    * Win32: libjpeg was updated.
    * Win32: libtiff was updated.
    * Win32: libpng was updated to 1.2.53.
    * Win32: OpenSSL was updated to v0.9.8zg.
    * Win32: included SSL certificates were updated.

(szptvlfn)