lex: fix buggy history

src/lex/mod.rs

@@ -117,8 +117,8 @@ impl Lexer {
     }
 
     pub fn prev(&mut self) -> Option<&Token> {
-        let prev = self.history.last()?;
         self.offset += 1;
+        let prev = &self.history[self.history.len() - self.offset];
         Some(prev)
     }