MdeModulePkg/DxeNetLib: Add array range check in NetIp6IsNetEqual().

MdeModulePkg/DxeNetLib: Add array range check in NetIp6IsNetEqual().

* The library API use array elements without any index range check, this
patch is to fix this issue to avoid null pointer reference.

V2
* Added an ASSERT check for the case PrefixLength equals to IP6_PREFIX_MAX.
* Synced some code descriptions to head file.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wang Fan <fan.wang@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>