Don't crash when asked if a client that has disconnected was local

ProcDRI2Dispatch uses LocalClient to determine if it's safe to respond
to a client that has made DRI2 requests which aren't sensible for
remote clients (anything but version). When the client has disappeared
mid-request stream (e.g. as a result of a kill -9, or a client-side
bug), LocalClient causes the X server to follow suit, as
((OsCommPtr)client->osPrivate)->trans_conn is NULL at this point.

The simple and obvious fix is to just return "not local" when
trans_conn is NULL, which fixes the crash I was seeing; however Keith
Packard pointed out that just checking trans_conn isn't enough;
quoting Keith:

"This looks almost right to me -- I reviewed the os code to see when
_XSERVTransClose is called (which is what frees the trans_conn data) and
found that every place which called that immediately set trans_conn to
NULL, except for the call in CloseDownFileDescriptor which is only
called from CloseDownConnection and which is immediately followed by
freeing the OsCommRec and setting client->osPrivate to NULL. So, I'd
suggest checking client->osPrivate in addition to the above check."

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

os/access.c

@@ -1123,6 +1123,11 @@ Bool LocalClient(ClientPtr client)
     pointer		addr;
     register HOST	*host;
 
+    if (!client->osPrivate)
+        return FALSE;
+    if (!((OsCommPtr)client->osPrivate)->trans_conn)
+        return FALSE;
+
     if (!_XSERVTransGetPeerAddr (((OsCommPtr)client->osPrivate)->trans_conn,
 	&notused, &alen, &from))
     {