Output: fix mode lifetime

Ensure we don't keep a pointer to an output mode owned by wlroots after
the wlr_output has been destroyed.

 river/Output.zig | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/river/Output.zig b/river/Output.zig
index d453df8..a69c9a4 100644
--- a/river/Output.zig
+++ b/river/Output.zig
@@ -230,6 +230,9 @@ fn handleDestroy(listener: *wl.Listener(*wlr.Output), wlr_output: *wlr.Output) v
 
     output.wlr_output = null;
     output.scene_output = null;
+    output.scheduled.mode = .none;
+    output.sent.mode = .none;
+    output.current.mode = .none;
     output.scheduled.state = .destroying;
 
     server.wm.dirtyWindowing();