USB: Make usb_buffer_free() NULL-safe

kfree() handles NULL arguments which is handy in error handling paths as one
does need to  insert bunch of ifs. How about making usb_buffer_free() do the
same?

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/usb.c

@@ -761,7 +761,9 @@ void usb_buffer_free (
 )
 {
 	if (!dev || !dev->bus || !dev->bus->op || !dev->bus->op->buffer_free)
-	    	return;
+		return;
+	if (!addr)
+		return;
 	dev->bus->op->buffer_free (dev->bus, size, addr, dma);
 }