openvpn: backport an upstream fix for a regression in using --cipher none (fixes #18676)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43823 3c298f89-4303-0410-b956-a3cf2f4a3e73