update comments and add assert for these files.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4750 6f19259b-4bc3-4df7-8a09-765794883524

MdePkg/Library/UefiUsbLib/UsbDxeLib.c

@@ -527,6 +527,10 @@ UsbClearEndpointHalt (
   EFI_USB_INTERFACE_DESCRIPTOR  InterfaceDescriptor;
   UINT8                         Index;
 
+  if (UsbIo == NULL) {
+    return EFI_INVALID_PARAMETER;
+  }
+
   ZeroMem (&EndpointDescriptor, sizeof (EFI_USB_ENDPOINT_DESCRIPTOR));
   //
   // First seach the endpoint descriptor for that endpoint addr