Added log to find about index overflow (#958)

## Describe your changes
Added log to find about index overflow.

pallets/ocex/src/validator.rs

@@ -756,7 +756,13 @@ impl<T: Config> Pallet<T> {
 	) -> Result<(), &'static str> {
 		let mut config = get_lmp_config(state, current_on_chain_epoch)?;
 		// We wrap around the index if we overflow
-		let next_index = config.index.checked_add(1).unwrap_or(0);
+		let next_index = match config.index.checked_add(1) {
+			None => {
+				log::error!(target:"ocex","Index overflow for LMPConfig {:?}", config.index);
+				0
+			},
+			Some(index) => index,
+		};
 		for (main, score) in scores {
 			store_q_score_and_uptime(
 				state,