Reset invalid log-reading position.

src/log.rs

@@ -43,6 +43,9 @@ fn read_log(
     log: Query<&Log, Changed<Log>>,
 ) -> Result<(), Box<dyn Error>> {
     for log in log.iter() {
+        if *position >= log.len() {
+            *position = 0;
+        }
         for (index, entry) in log.iter().enumerate() {
             if index >= *position {
                 tts.speak(entry.message.clone(), false)?;