[util] Avoid integer overflow in CheckDiskSpace

Author: dergoegge

src/init.cpp

@@ -1631,10 +1631,11 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
 
     // On first startup, warn on low block storage space
     if (!fReindex && !fReindexChainState && chain_active_height <= 1) {
+        uint64_t assumed_chain_bytes{chainparams.AssumedBlockchainSize() * 1024 * 1024 * 1024};
         uint64_t additional_bytes_needed{
             chainman.m_blockman.IsPruneMode() ?
-                chainman.m_blockman.GetPruneTarget() :
-                chainparams.AssumedBlockchainSize() * 1024 * 1024 * 1024};
+                std::min(chainman.m_blockman.GetPruneTarget(), assumed_chain_bytes) :
+                assumed_chain_bytes};
 
         if (!CheckDiskSpace(args.GetBlocksDirPath(), additional_bytes_needed)) {
             InitWarning(strprintf(_(