MdeModulePkg/NetLib: Handle an invalid IPv6 address case

MdeModulePkg/NetLib: Handle an invalid IPv6 address case

Handle an invalid IPv6 address in NetLibAsciiStrToIp6(),
like '2000:aaaa::1com'.

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