Refine code to make it more safely.

Signed-off-by: Dong Eric <eric.dong@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13931 6f19259b-4bc3-4df7-8a09-765794883524

MdeModulePkg/Universal/Network/IScsiDxe/IScsiCHAP.c

@@ -332,6 +332,9 @@ IScsiCHAPToSendReq (
   Session     = Conn->Session;
   AuthData    = &Session->AuthData;
   LoginReq    = (ISCSI_LOGIN_REQUEST *) NetbufGetByte (Pdu, 0, 0);
+  if (LoginReq == NULL) {
+    return EFI_PROTOCOL_ERROR;
+  }
   Status      = EFI_SUCCESS;
 
   RspLen      = 2 * ISCSI_CHAP_RSP_LEN + 3;