Avoid divide-by-zero in header sync logs when NodeClock is behind

src/validation.cpp

@@ -4189,6 +4189,7 @@ bool ChainstateManager::ProcessNewBlockHeaders(const std::vector<CBlockHeader>&
         if (IsInitialBlockDownload() && ppindex && *ppindex) {
             const CBlockIndex& last_accepted{**ppindex};
             int64_t blocks_left{(NodeClock::now() - last_accepted.Time()) / GetConsensus().PowTargetSpacing()};
+            blocks_left = std::max<int64_t>(0, blocks_left);
             const double progress{100.0 * last_accepted.nHeight / (last_accepted.nHeight + blocks_left)};
             LogInfo("Synchronizing blockheaders, height: %d (~%.2f%%)\n", last_accepted.nHeight, progress);
         }
@@ -4215,6 +4216,7 @@ void ChainstateManager::ReportHeadersPresync(const arith_uint256& work, int64_t
     GetNotifications().headerTip(GetSynchronizationState(initial_download), height, timestamp, /*presync=*/true);
     if (initial_download) {
         int64_t blocks_left{(NodeClock::now() - NodeSeconds{std::chrono::seconds{timestamp}}) / GetConsensus().PowTargetSpacing()};
+        blocks_left = std::max<int64_t>(0, blocks_left);
         const double progress{100.0 * height / (height + blocks_left)};
         LogInfo("Pre-synchronizing blockheaders, height: %d (~%.2f%%)\n", height, progress);
     }