MdeModulePkg/DxeHttpLib: Add boundary condition check.

MdeModulePkg/DxeHttpLib: Add boundary condition check.

v2:
* Fix GCC the build error.

This patch is to add the boundary condition check to make sure
the accessed buffer is valid.

Cc: Gary Lin <glin@suse.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wang Fan <fan.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Tested-by: Gary Lin <glin@suse.com>