parser: add missing level guard in if let

Resolves <https://issues.oss-fuzz.com/issues/393401467>

Author: Kijewski

rinja_parser/src/node.rs

@@ -426,6 +426,7 @@ impl<'a> CondTest<'a> {
                 let mut expr = Expr::parse(i, s.level, false)?;
                 if let Expr::BinOp(_, _, ref mut right) = expr.inner {
                     if matches!(right.inner, Expr::Var("set" | "let")) {
+                        let _level_guard = s.level.nest(i)?;
                         *i = right.span.as_suffix_of(start).unwrap();
                         let start_span = Span::from(*i);
                         let new_right = Self::parse_cond(i, s)?;

rinja_parser/src/tests.rs

@@ -1288,3 +1288,14 @@ fn test_generics_parsing() {
     )
     .unwrap();
 }
+
+#[test]
+fn fuzzed_deeply_tested_if_let() {
+    let src = include_str!("../tests/fuzzed-deeply-tested-if-let.txt");
+    let syntax = Syntax::default();
+    let err = Ast::from_str(src, None, &syntax).unwrap_err();
+    assert_eq!(
+        err.to_string().lines().next(),
+        Some("your template code is too deeply nested, or the last expression is too complex"),
+    );
+}