ipc: Fix NULL check stopping main app visibility events from working

doc/changes/ipc/mr.721.md

@@ -2,6 +2,7 @@
 - mr.754
 - mr.768
 - mr.800
+- mr.846
 ---
 Now that there is a interface that allows the compositor to support
 multi-client rendering use that instead of doing our own rendering.

src/xrt/ipc/server/ipc_server_process.c

@@ -525,8 +525,8 @@ handle_overlay_client_events(volatile struct ipc_client_state *ics, int active_i
 		return;
 	}
 
-	// Does this client have a compositor yet?
-	if (ics->xc) {
+	// Does this client have a compositor yet, if not return?
+	if (ics->xc == NULL) {
 		return;
 	}