solaris/open_tun: prevent crash when dev is empty string

solaris/open_tun: prevent crash when dev is empty string

This was originally reported on GH, but never dealt with.
Make sure 'ptr' is always initialized to prevent derefence of null
pointer in case of empty dev string.

While at it, change the if condition to use ptr instead of dev, since
dev is not used anymore in the logic.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220917125811.13549-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25235.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>