NetworkPkg: Add more parameter or return status check in UDP6 driver

NetworkPkg: Add more parameter or return status check in UDP6 driver

In UDP6Dxe, there are several places that may be enhanced
to check input parameters and returned status. This patch
is to fix these issues.

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>