RPC/Mempool: Avoid extra decrement of unsigned below 0 when building fee histogram

Author: luke-jr

src/rpc/mempool.cpp

@@ -709,7 +709,8 @@ UniValue MempoolInfoToJSON(const CTxMemPool& pool, const std::optional<MempoolHi
             const CAmount fee_rate{std::max(std::min(dfpb, tfpb), std::min(fpb, afpb))};
 
             // Distribute fee rates
-            for (size_t i = floors.size(); i-- > 0;) {
+            for (size_t i = floors.size(); i > 0;) {
+                --i;
                 if (fee_rate >= floors[i]) {
                     sizes[i] += size;
                     ++count[i];