Check for NULL before calling memcpy().

Although it works without problem, it is technically illegal to call memcpy()
with a destination of NULL, even if size is zero.

Reported-by: Ludovic Rousseau <ludovic.rousseau@gmail.com>

linux/hid-libusb.c

@@ -706,7 +706,8 @@ static int return_data(hid_device *dev, unsigned char *data, size_t length)
 	   return buffer (data), and delete the liked list item. */
 	struct input_report *rpt = dev->input_reports;
 	size_t len = (length < rpt->len)? length: rpt->len;
-	memcpy(data, rpt->data, len);
+	if (len > 0)
+		memcpy(data, rpt->data, len);
 	dev->input_reports = rpt->next;
 	free(rpt->data);
 	free(rpt);