Xext: free the XvRTVideoNotify when turning off from the same client

This fixes a use-after-free bug:

When a client first calls XvdiSelectVideoNotify() on a drawable with a
TRUE onoff argument, a struct XvVideoNotifyRec is allocated. This struct
is added twice to the resources:
  - as the drawable's XvRTVideoNotifyList. This happens only once per
    drawable, subsequent calls append to this list.
  - as the client's XvRTVideoNotify. This happens for every client.

The struct keeps the ClientPtr around once it has been added for a
client. The idea, presumably, is that if the client disconnects we can remove
all structs from the drawable's list that match the client (by resetting
the ClientPtr to NULL), but if the drawable is destroyed we can remove
and free the whole list.

However, if the same client then calls XvdiSelectVideoNotify() on the
same drawable with a FALSE onoff argument, only the ClientPtr on the
existing struct was set to NULL. The struct itself remained in the
client's resources.

If the drawable is now destroyed, the resource system invokes
XvdiDestroyVideoNotifyList which frees the whole list for this drawable
- including our struct. This function however does not free the resource
for the client since our ClientPtr is NULL.

Later, when the client is destroyed and the resource system invokes
XvdiDestroyVideoNotify, we unconditionally set the ClientPtr to NULL. On
a struct that has been freed previously. This is generally frowned upon.

Fix this by calling FreeResource() on the second call instead of merely
setting the ClientPtr to NULL. This removes the struct from the client
resources (but not from the list), ensuring that it won't be accessed
again when the client quits.

Note that the assignment tpn->client = NULL; is superfluous since the
XvdiDestroyVideoNotify function will do this anyway. But it's left for
clarity and to match a similar invocation in XvdiSelectPortNotify.

CVE-2022-46342, ZDI-CAN 19400

This vulnerability was discovered by:
Jan-Niklas Sohn working with Trend Micro Zero Day Initiative

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Olivier Fourdan <ofourdan@redhat.com>