NetworkPkg/DnsDxe: Fixed the assert issue in DnsDxe

Fix the DnsDxe assert issue when the incorrect answer message
received.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Zhang Lubo <lubo.zhang@intel.com>