net: optimize compact block extra tx iteration

`vExtraTxnForCompact` was resized to its configured capacity on the first insertion.
Before the ring was filled, compact block reconstruction scanned default `{Wtxid::ZERO, nullptr}` entries created for unused slots.

Reserve the configured capacity and append entries until the cache is full, then keep the same ring-buffer overwrite behavior.
This preserves the configured maximum and replacement order while keeping reconstruction from scanning unused capacity.

Author: l0rinc

src/net_processing.cpp

@@ -1907,11 +1907,13 @@ std::vector<CTransactionRef> PeerManagerImpl::AbortPrivateBroadcast(const uint25
 
 void PeerManagerImpl::AddToCompactExtraTransactions(const CTransactionRef& tx)
 {
-    if (m_opts.max_extra_txs <= 0)
-        return;
-    if (!vExtraTxnForCompact.size())
-        vExtraTxnForCompact.resize(m_opts.max_extra_txs);
-    vExtraTxnForCompact[vExtraTxnForCompactIt] = std::make_pair(tx->GetWitnessHash(), tx);
+    if (m_opts.max_extra_txs == 0) return;
+    if (vExtraTxnForCompact.size() < m_opts.max_extra_txs) {
+        if (vExtraTxnForCompact.empty()) vExtraTxnForCompact.reserve(m_opts.max_extra_txs);
+        vExtraTxnForCompact.emplace_back(tx->GetWitnessHash(), tx);
+    } else {
+        vExtraTxnForCompact[vExtraTxnForCompactIt] = std::make_pair(tx->GetWitnessHash(), tx);
+    }
     vExtraTxnForCompactIt = (vExtraTxnForCompactIt + 1) % m_opts.max_extra_txs;
 }