NetworkPkg: update code for NULL pointer check.

NetworkPkg: update code for NULL pointer check.

This patch updates the HTTP driver to initialize the local variable for NULL
and check the NULL pointer before dereference it.

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