fix panics with misformed demos and incorrect length values

fuzz/fuzz_targets/fuzz_target_1.rs

@@ -0,0 +1,13 @@
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+pub use tf_demo_parser::{Demo, DemoParser, Parse, ParseError, ParserState, Stream};
+
+fn fuzz(data: &[u8]) {
+    let data = data.to_vec();
+    let demo = Demo::new(data);
+    let parser = DemoParser::new_all(demo.get_stream());
+    let _ = parser.parse();
+}
+
+fuzz_target!(|data: &[u8]| { fuzz(data) });