nbd: fix uninitialized variable warning

nbd: fix uninitialized variable warning

Fixes:
/mnt/sdb/qemu/nbd/server.c: In function 'nbd_handle_request':
/mnt/sdb/qemu/nbd/server.c:2313:9: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     int ret;

Reported-by: Euler Robot <[email protected]>
Signed-off-by: Pan Nengyuan <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Message-Id: <20200108025132[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>