fix uninitialized ip6 keys within session.c

fix uninitialized ip6 keys within session.c

fix for functions:
- stream_session_lookup_listener6
- stream_session_lookup6
- stream_session_lookup_transport6
- stream_session_connect_notify

eg.
~/vpp/build-data/../src/vnet/session/session.c: In function 'stream_session_lookup_listener6':
~/vpp/build-data/../src/vnet/session/session.c:222:17: error: 'kv6.key[3]' is used uninitialized in this function [-Werror=uninitialized]
   session_kv6_t kv6;
                 ^
~/vpp/build-data/../src/vnet/session/session.c:222:17: error: 'kv6.key[2]' is used uninitialized in this function [-Werror=uninitialized]
~/vpp/build-data/../src/vnet/session/session.c:222:17: error: 'kv6.key[5]' is used uninitialized in this function [-Werror=uninitialized]
~/vpp/build-data/../src/vnet/session/session.c:222:17: error: 'kv6.key[4]' is used uninitialized in this function [-Werror=uninitialized]

Change-Id: I221cafbaf4effa43662ffcc7bf5f9581e28f9db9
Signed-off-by: Gabriel Ganne <gabriel.ganne@enea.com>