solana: Block redeem when there are no transceivers

Author: johnsaigle

solana/programs/example-native-token-transfers/src/instructions/redeem.rs

@@ -24,6 +24,8 @@ pub struct Redeem<'info> {
     // NOTE: this works when the contract is paused
     #[account(
         constraint = config.threshold > 0 @ NTTError::ZeroThreshold,
+        constraint = config.next_transceiver_id != 0 @ NTTError::NoRegisteredTransceivers,
+        constraint = config.enabled_transceivers.is_empty() @ NTTError::NoRegisteredTransceivers,
     )]
     pub config: Account<'info, Config>,