MdeModulePkg: Freed packet buffer when error occurs to avoid memory leak.

MdeModulePkg: Freed packet buffer when error occurs to avoid memory leak.

* In function Mtftp4WrqSendBlock(), when packet is not needed, function
  returns EFI_ABORTED but not freed the packet buffer. It results some
  memory leak and this patch is to fix this issue.

Cc: Jiaxin Wu <jiaxin.wu@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: Wang Fan <fan.wang@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>