NetworkPkg: Fix potentially uninitialized local variable

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: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>

NetworkPkg/HttpDxe/HttpProto.c

@@ -1475,6 +1475,8 @@ HttpTransmitTcp (
 
   Status                = EFI_SUCCESS;
   Buffer                = NULL;
+  TempFragment.Len      = 0;
+  TempFragment.Bulk     = NULL;
 
   //
   // Need to encrypt data.