XkbBinding: restore assertion in pressed()

This assertion was removed by accident in the previous commit.

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

diff --git a/river/XkbBinding.zig b/river/XkbBinding.zig
index 2a278ff..c0866da 100644
--- a/river/XkbBinding.zig
+++ b/river/XkbBinding.zig
@@ -137,6 +137,9 @@ fn handleRequest(
 
 pub fn pressed(binding: *XkbBinding) void {
     assert(!binding.sent_pressed);
+    // Input event processing should not continue after a state change
+    // until that event is sent to the window manager in an update and acked.
+    assert(binding.wm_scheduled.state_change == .none);
     binding.wm_scheduled.state_change = .pressed;
     server.wm.dirtyWindowing();
 }