BUG/MEDIUM: connection: fix proxy v2 header again!

Last commit 77d1f01 ("BUG/MEDIUM: connection: fix memory corruption
when building a proxy v2 header") was wrong, using &cn_trash instead
of cn_trash resulting in a warning and the client's SSL cert CN not
being stored at the proper location.

Thanks to Lukas Tribus for spotting this quickly.

This should be backported to 1.5 after the patch above is backported.
(cherry picked from commit 3b9a0c9d4d083d749846d66f9bd4caabafe4ee78)