Reject TXs sent via RPC with no signatures

(cherry picked from commit a888f2f516)

core/src/rpc.rs

@@ -1138,6 +1138,9 @@ fn _send_transaction(
     wire_transaction: Vec<u8>,
     last_valid_slot: Slot,
 ) -> Result<String> {
+    if transaction.signatures.is_empty() {
+        return Err(RpcCustomError::SendTransactionIsNotSigned.into());
+    }
     let signature = transaction.signatures[0];
     meta.send_transaction_service
         .send(signature, wire_transaction, last_valid_slot);