tun.c: make wintun_register_ring_buffer() non-fatal on failures

tun.c: make wintun_register_ring_buffer() non-fatal on failures

Wintun allows multiple handles to be opened on it's NDIS device pipe.
Just by succeeding to open the pipe does not warrant the adapter is
unused.

When iterating for available Wintun adapter, we will need to try
registering ring buffers with each one to actually determine which one
is used and which one is not.

Therefore, a failure to register ring buffers should be detectable, but
not M_FATAL.

Signed-off-by: Simon Rozman <simon@rozman.si>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <20191220161117.1434-3-simon@rozman.si>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg19283.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>