BUG/MAJOR: ns: HAProxy segfault if the cli_conn is not from a network connection

The path "MAJOR: namespace: add Linux network namespace support" doesn't
permit to use internal data producer like a "peers synchronisation"
system. The result is a segfault when the internal application starts.

This patch fix the commit b3e54fe387c7c1ea750f39d3029672d640c499f9
It is introduced in 1.6dev version, it doesn't need to be backported.