os/xdmcp: Don't create a new socket in XdmcpReset()

xdmcpSocket survives during the reset, there is no
need to create a new one.

This commit restores logic that was broken by
49c0f241 in Xorg 1.19.

Signed-off-by: Alexander Volkov <a.volkov@rusbitech.ru>
(cherry picked from commit 32677ce0)