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/HttpImpl.c

@@ -591,10 +591,12 @@ EfiHttpRequest (
 
   Status = HttpGenRequestMessage (HttpMsg, FileUrl, &RequestMsg, &RequestMsgSize);
 
-  if (EFI_ERROR (Status)) {
+  if (EFI_ERROR (Status) || NULL == RequestMsg) {
     goto Error3;
   }
 
+  ASSERT (RequestMsg != NULL);
+
   //
   // Every request we insert a TxToken and a response call would remove the TxToken.
   // In cases of PUT/POST, after an initial request-response pair, we would do a