Fix a panic when parsing var patterns

cranelift/isle/fuzz/fuzz_targets/parse.rs

@@ -3,8 +3,15 @@
 use libfuzzer_sys::fuzz_target;
 
 fuzz_target!(|s: &str| {
-    if let Ok(lexer) = isle::lexer::Lexer::from_str(s, "fuzz-input.isle") {
+    let _ = env_logger::try_init();
+
+    let lexer = isle::lexer::Lexer::from_str(s, "fuzz-input.isle");
+    log::debug!("lexer = {:?}", lexer);
+
+    if let Ok(lexer) = lexer {
         let mut parser = isle::parser::Parser::new(lexer);
-        let _ = parser.parse_defs();
+
+        let defs = parser.parse_defs();
+        log::debug!("defs = {:?}", defs);
     }
 });