usbredir: fix free call

data might point into the middle of a larger buffer, there is a separate
free_on_destroy pointer passed into bufp_alloc() to handle that.  It is
only used in the normal workflow though, not when dropping packets due
to the queue being full.  Fix that.

Resolves: #491

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210722072756.647673-1-kraxel@redhat.com>

hw/usb/redirect.c

@@ -476,7 +476,7 @@ static int bufp_alloc(USBRedirDevice *dev, uint8_t *data, uint16_t len,
     if (dev->endpoint[EP2I(ep)].bufpq_dropping_packets) {
         if (dev->endpoint[EP2I(ep)].bufpq_size >
                 dev->endpoint[EP2I(ep)].bufpq_target_size) {
-            free(data);
+            free(free_on_destroy);
             return -1;
         }
         dev->endpoint[EP2I(ep)].bufpq_dropping_packets = 0;