notes: check error code returned by `git_iterator_advance`

When calling `git_note_next`, we end up calling `git_iterator_advance`
but ignore its error code. The intent is that we do not want to return
an error if it returns `GIT_ITEROVER`, as we want to return that value
on the next invocation of `git_note_next`. We should still check for any
other error codes returned by `git_iterator_advance` to catch unexpected
internal errors.

Fix this by checking the function's return value, ignoring
`GIT_ITEROVER`.

src/notes.c

@@ -808,8 +808,11 @@ int git_note_next(
 
 	git_oid_cpy(note_id, &item->id);
 
-	if (!(error = process_entry_path(item->path, annotated_id)))
-		git_iterator_advance(NULL, it);
+	if ((error = process_entry_path(item->path, annotated_id)) < 0)
+		return error;
 
-	return error;
+	if ((error = git_iterator_advance(NULL, it)) < 0 && error != GIT_ITEROVER)
+		return error;
+
+	return 0;
 }