input: Fix crash for weird bracketed paste

I can reproduce by pasting after

```fish
echo \cc foo | fish_clipboard_copy
```

in Wezterm

src/input.rs

@@ -463,7 +463,9 @@ fn paste_is_buffering(&self) -> bool {
     }
     fn paste_commit(&mut self) {
         self.push_front(CharEvent::from_readline(ReadlineCmd::EndUndoGroup));
-        let buffer = self.paste_buffer.take().unwrap();
+        let Some(buffer) = self.paste_buffer.take() else {
+            return;
+        };
         self.push_front(CharEvent::Command(sprintf!(
             "__fish_paste %s",
             escape(&str2wcstring(&buffer))