bridge: fdb get: add missing json init (new_json_obj)

bridge: fdb get: add missing json init (new_json_obj)

'bridge fdb get' has json support but the json object is never initialized

before patch:

$ bridge -j fdb get 56:23:28:4f:4f:e5 dev vx0
56:23:28:4f:4f:e5 dev vx0 master br0 permanent
$

after patch:

$ bridge -j fdb get 56:23:28:4f:4f:e5 dev vx0 | \
python -c \
'import sys,json;print(json.dumps(json.loads(sys.stdin.read()),indent=4))'
[
    {
        "master": "br0",
        "mac": "56:23:28:4f:4f:e5",
        "flags": [],
        "ifname": "vx0",
        "state": "permanent"
    }
]
$

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>