NetworkPkg: Fix the potential NULL pointer dereferenced issue

NetworkPkg: Fix the potential NULL pointer dereferenced issue

This patch is used to fix the potential NULL pointer dereferenced
in function 'ParseDnsResponse'.

Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Zhang Lubo <lubo.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Reviewed-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19178 6f19259b-4bc3-4df7-8a09-765794883524