Fix unintialized variable in plugin_vlog()

Fix unintialized variable in plugin_vlog()

Does not seem to be a problem in normal usage, because one of the PLOG_ERR,
PLOG_WARN, PLOG_NOTE or PLOG_DEBUG will be set, and will cause msg_flags
to be initialized.  In the worst case, msg_flags might accidentally
end up having M_FATAL set, causing openvpn to exit.

This was previously fixed in the master branch (5ead2ae0), but was not
backported to release/2.3, probably because that commit fixed other parts
of the code too, and those parts are quite different between master and
release/2.3.

Re-discovered by coverity.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1446984267-3323-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10450
Signed-off-by: Gert Doering <gert@greenie.muc.de>