Fix underflow in recursion limit code

The recursion limiter didn't prevent underflow. Fixed properly
and also switched to `Cell` because atomic support seems fairly
variable.

src/parser.rs

@@ -24,7 +24,7 @@ use core::fmt;
 #[cfg(feature = "std")]
 use std::rc::Rc;
 #[cfg(feature = "std")]
-use std::sync::atomic::{AtomicUsize, Ordering};
+use std::cell::Cell;
 
 use log::debug;
 #[cfg(feature = "std")]
@@ -62,11 +62,12 @@ macro_rules! return_ok_if_some {
 macro_rules! check_recursion_depth {
     ($this:ident) => {
         let remaining_depth = $this.remaining_depth.clone();
-        if remaining_depth.fetch_sub(1, Ordering::SeqCst) <= 0 {
+        remaining_depth.set(remaining_depth.get().saturating_sub(1));
+        if remaining_depth.get() == 0 {
             return Err(ParserError::RecursionLimitExceeded);
         }
         defer! {
-            remaining_depth.fetch_add(1, Ordering::SeqCst);
+            remaining_depth.set(remaining_depth.get() + 1);
         }
     };
 }
@@ -136,7 +137,7 @@ pub struct Parser<'a> {
     index: usize,
     dialect: &'a dyn Dialect,
     #[cfg(feature = "std")]
-    remaining_depth: Rc<AtomicUsize>,
+    remaining_depth: Rc<Cell<usize>>,
 }
 
 impl<'a> Parser<'a> {
@@ -147,7 +148,7 @@ impl<'a> Parser<'a> {
             index: 0,
             dialect,
             #[cfg(feature = "std")]
-            remaining_depth: Rc::new(AtomicUsize::new(96)),
+            remaining_depth: Rc::new(Cell::new(96)),
         }
     }