Panic: unbalanced environment stack or state stack

Hi, with some random typing, i found `push_mathml` crashes with inputting a macro like `\bar` (which takes some arguments) and a subscript ``` rust mod tests { #[test] fn should_not_panic() { use pulldown_latex::{Parser, RenderConfig, Storage}; let storage = Storage::new(); let parser = Parser::new("\\bar_a", &storage); let config = RenderConfig::default(); let mut math_output = String::new(); let res = pulldown_latex::push_mathml(&mut math_output, parser, config); assert!(res.is_ok()); } } ``` This code sinppet panics: ``` thread 'cmark::latex::tests::should_not_panic' panicked at /.../pulldown-latex-0.7.1/src/mathml.rs:913:13: unbalanced environment stack or state stack ``` And it's reproducible on the [demo page](https://carloskiki.github.io/pulldown-latex/).