fix build failure in nbd_read_reply_entry()

travis builds fail at HEAD at rc3 master with

  block/nbd-client.c: In function ‘nbd_read_reply_entry’:
  block/nbd-client.c:110:8: error: ‘ret’ may be used uninitialized in this function [-Werror=uninitialized]

fix it by initializing 'ret' to 0

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
