BUG/MEDIUM: send_proxy: fix initialisation of send_proxy_ofs

BUG/MEDIUM: send_proxy: fix initialisation of send_proxy_ofs

Commit b22e55bc introduced send_proxy_ofs but forgot to initialize it,
which remained unnoticed since it's always at the same place in the
stream interface. On a machine with dirty RAM returned by malloc(),
some responses were holding a PROXY header, which normally is not
possible.

The problem goes away after properly initializing the field upon each
new session_accept().

This fix does not need to be backported except if any code makes use of
a backport of this feature.