BUG/MINOR: namespace: avoid closing fd when socket failed in my_socketat

BUG/MINOR: namespace: avoid closing fd when socket failed in my_socketat

we cannot return right after socket opening as we need to move back to
the default namespace first

this should fix github issue #500

this might be backported to all version >= 1.6

Fixes: b3e54fe387c7c1 ("MAJOR: namespace: add Linux network namespace
support")
Signed-off-by: William Dauchy <w.dauchy@criteo.com>
(cherry picked from commit f7dcdc8a6f63b172360019beb6e1e22e7c98f17d)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>