afb-cred: fix default values

afb-cred: fix default values

Because SO_PEERCRED returns without error even when no data
is available (tcp by example), the resulting uid is now tested.

Also, for tcp, by default avoid by default to create a default
user value. Instead, return NULL. This will allow client having
an HTTP/Websocket connection to get full rights on the binder.

Change-Id: I2defb585bf79c023e2391c2e18d6de17e5112770
Signed-off-by: José Bollo <jose.bollo@iot.bzh>