MdeModulePkg/DxeHttpLib: Avoid the pointless comparison of UINTN with zero

UINTN is unsigned integer, so it's pointless to compare it with zero.

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