rpc: Fixed signed integer overflow for large feerates

test/functional/mempool_accept.py

@@ -90,9 +90,17 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
         txid_in_block = self.wallet.sendrawtransaction(from_node=node, tx_hex=raw_tx_in_block)
         self.generate(node, 1)
         self.mempool_size = 0
+        # Also check feerate. 1BTC/kvB fails
+        assert_raises_rpc_error(-8, "Fee rates larger than or equal to 1BTC/kvB are not accepted", lambda: self.check_mempool_result(
+            result_expected=None,
+            rawtxs=[raw_tx_in_block],
+            maxfeerate=1,
+        ))
+        # ... 0.99 passes
         self.check_mempool_result(
             result_expected=[{'txid': txid_in_block, 'allowed': False, 'reject-reason': 'txn-already-known'}],
             rawtxs=[raw_tx_in_block],
+            maxfeerate=0.99,
         )
 
         self.log.info('A transaction not in the mempool')