NetworkPkg/Ip6Dxe: Fix ASSERT logic in Ip6ProcessRouterAdvertise()

NetworkPkg/Ip6Dxe: Fix ASSERT logic in Ip6ProcessRouterAdvertise()

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2655

This patch fixes reversed logic of recently added ASSERTs which should
ensure that Ip6IsNDOptionValid() implementation properly reacts to invalid
packets.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Signed-off-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Reviewed-by: Siyuan Fu <siyuan.fu@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Fixes: 9c20342eed70ec99ec50cd73cb81804299f05403