bgpd: fix default instance name when un-hiding

bgpd: fix default instance name when un-hiding

When unconfiguring a default BGP instance with VPN SAFI configurations,
the default BGP structure remains but enters a hidden state. Upon
reconfiguration, the instance name incorrectly appears as "VIEW ?"
instead of "VRF default". And the name_pretty pointer

The name_pretty pointer is replaced by another one with the incorrect
name. This also leads to a memory leak as the previous pointer is not
properly freed.

Do not rewrite the instance name.

Fixes: 4d0e7a49cf ("bgpd: VRF-Lite fix default bgp delete")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>