Only parse footnote definitions if they target a reference that exists.

src/intermediate/registry.rs

@@ -99,6 +99,16 @@ pub(crate) async fn register_footnote_definition<'orig, 'parse>(
     label: &'parse str,
     definition: &'orig Vec<Element<'parse>>,
 ) -> Result<(), CustomError> {
+    let has_existing: bool = {
+        let mut registry = registry.lock().unwrap();
+        registry
+            .footnote_ids
+            .iter_mut()
+            .any(|(id, _definition)| *id == Some(label))
+    };
+    if !has_existing {
+        return Ok(());
+    }
     let contents = convert_definition_contents(registry.clone(), definition).await?;
     let mut registry = registry.lock().unwrap();
     if let Some((_existing_id, existing_definition)) = registry