in -current this library is provided by the libffi package. But the shared version previously included in gcc-java has been removed, see changelog dated in Mon Jun 25 05:17:48 UTC 2012.
My guess is that somehow slapt-get misbehaved about this update.
Anyhow slapt-get's usage is *not* recommended to upgrade Slackware, better use slackpkg instead. Let me quote "man slackpkg"
Slackpkg can be used to upgrade the whole distribution.
The usual way is to do:
# slackpkg update
# slackpkg install-new
# slackpkg upgrade-all
# slackpkg clean-system
For now, check if libffi is installed (version included in -current and if not, install it.
EDIT audriusk was faster than me...