record: Fix undefined memcpy in RecordAClientStateChange

From Adam Jackson
f44ac101c523a0439bd1a864850e3c1a4e154549 in xserver git

avoids a large number of malloc(0) calls
ok deraadt@ who had almost the same diff

xserver/record/record.c

@@ -2728,7 +2728,8 @@ RecordAClientStateChange(CallbackListPtr *pcbl, void *nulldata,
     case ClientStateRetained:  /* client disconnected */
 
         /* RecordDisableContext modifies contents of ppAllContexts. */
-        numContextsCopy = numContexts;
+        if (!(numContextsCopy = numContexts))
+            break;
         ppAllContextsCopy = xallocarray(numContextsCopy,
                                         sizeof(RecordContextPtr));
         assert(ppAllContextsCopy);