Merge bitcoin#30544: rpc: fix maybe-uninitialized compile warning in getchaintxstats

2e86f2b rpc: fix maybe-uninitialized compile warning in getchaintxstats (Michael Dietz)

Pull request description:

  This resolves the compiler warning about potential uninitialized use of window_tx_count introduced in fa2dada.

  The warning:
  ```
  CXX      rpc/libbitcoin_node_a-blockchain.o
  rpc/blockchain.cpp: In function ‘getchaintxstats()::<lambda(const RPCHelpMan&, const JSONRPCRequest&)>’:
  rpc/blockchain.cpp:1742:38: warning: ‘*(std::_Optional_payload_base<unsigned int>::_Storage<unsigned int, true>*)((char*)&window_tx_count + offsetof(const std::optional<unsigned int>,std::optional<unsigned int>::<unnamed>.std::_Optional_base<unsigned int, true, true>::<unnamed>)).std::_Optional_payload_base<unsigned int>::_Storage<unsigned int, true>::_M_value’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   1742 |                 ret.pushKV("txrate", double(*window_tx_count) / nTimeDiff);
        |
  ```

ACKs for top commit:
  maflcko:
    lgtm ACK 2e86f2b
  theStack:
    ACK 2e86f2b
  tdb3:
    ACK 2e86f2b

Tree-SHA512: c087e8f1cd68dd8df734a8400d30a95abe57ebd56cd53aef4230e425b33a23aa55b3af42abfd162e3be8c937a4c27e56abb70a4fedb10e2df64d52d577e0f262

Author: fanquake

src/rpc/blockchain.cpp

@@ -1722,9 +1722,6 @@ static RPCHelpMan getchaintxstats()
 
     const CBlockIndex& past_block{*CHECK_NONFATAL(pindex->GetAncestor(pindex->nHeight - blockcount))};
     const int64_t nTimeDiff{pindex->GetMedianTimePast() - past_block.GetMedianTimePast()};
-    const auto window_tx_count{
-        (pindex->nChainTx != 0 && past_block.nChainTx != 0) ? std::optional{pindex->nChainTx - past_block.nChainTx} : std::nullopt,
-    };
 
     UniValue ret(UniValue::VOBJ);
     ret.pushKV("time", (int64_t)pindex->nTime);
@@ -1736,10 +1733,11 @@ static RPCHelpMan getchaintxstats()
     ret.pushKV("window_block_count", blockcount);
     if (blockcount > 0) {
         ret.pushKV("window_interval", nTimeDiff);
-        if (window_tx_count) {
-            ret.pushKV("window_tx_count", *window_tx_count);
+        if (pindex->nChainTx != 0 && past_block.nChainTx != 0) {
+            const auto window_tx_count = pindex->nChainTx - past_block.nChainTx;
+            ret.pushKV("window_tx_count", window_tx_count);
             if (nTimeDiff > 0) {
-                ret.pushKV("txrate", double(*window_tx_count) / nTimeDiff);
+                ret.pushKV("txrate", double(window_tx_count) / nTimeDiff);
             }
         }
     }