NetworkPkg: Add ASSERT error handling for UDP6 driver

NetworkPkg: Add ASSERT error handling for UDP6 driver

In Udp6Dxe, there are several places use ASSERT to check returned
value. But these errors should be handled if they occur, this patch
is to fix this issue.

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