[dovecot/core] ba4eb9: lib-ssl-iostream: Fix compiler warning

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: ba4eb91d932ee5eb1c3e2dafd3c0e3e3659d299c
      https://github.com/dovecot/core/commit/ba4eb91d932ee5eb1c3e2dafd3c0e3e3659d299c
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-07-10 (Mon, 10 Jul 2017)

  Changed paths:
    M src/lib-ssl-iostream/iostream-openssl-context.c

  Log Message:
  -----------
  lib-ssl-iostream: Fix compiler warning

~SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS makes it unsigned, so it doesn't fit
into signed 32bit long.