NetworkPkg: Fix a memory leak issue in UDP6 driver

NetworkPkg: Fix a memory leak issue in UDP6 driver

In UDP6Dxe Udp6Groups(), the code return directly without free the
buffer allocated for McastIp when JoinFlag is TRUE. It is a memory
leak issue that needs to be fixed. 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>