NetworkPkg/HttpDxe: Fix the potential NULL dereference

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Hao A <hao.a.wu@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>

NetworkPkg/HttpDxe/HttpProto.c

@@ -1655,6 +1655,8 @@ HttpTcpTransmit (
   UINTN                     UrlSize;
   UINTN                     RequestMsgSize;
 
+  RequestMsg = NULL;
+
   ValueInItem = (HTTP_TOKEN_WRAP *) Item->Value;
   if (ValueInItem->TcpWrap.IsTxDone) {
     return EFI_SUCCESS;
@@ -1682,10 +1684,12 @@ HttpTcpTransmit (
                  );
   FreePool (Url);
 
-  if (EFI_ERROR (Status)){
+  if (EFI_ERROR (Status) || NULL == RequestMsg){
     return Status;
   }
 
+  ASSERT (RequestMsg != NULL);
+
   //
   // Transmit the request message.
   //