NetworkPkg: Replace ASSERT with error handling in Http boot and IScsi

NetworkPkg: Replace ASSERT with error handling in Http boot and IScsi

v2:
*Fix some memory leak issue.

This patch is used to replace ASSERT with error handling in Http boot
Driver and IScsi driver.

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