Check pindexBestForkBase for null

src/main.cpp

@@ -1211,13 +1211,13 @@ void CheckForkWarningConditions()
 
     if (pindexBestForkTip || (pindexBestInvalid && pindexBestInvalid->nChainWork > chainActive.Tip()->nChainWork + (chainActive.Tip()->GetBlockWork() * 6)))
     {
-        if (!fLargeWorkForkFound)
+        if (!fLargeWorkForkFound && pindexBestForkBase)
         {
             std::string warning = std::string("'Warning: Large-work fork detected, forking after block ") +
                 pindexBestForkBase->phashBlock->ToString() + std::string("'");
             CAlert::Notify(warning, true);
         }
-        if (pindexBestForkTip)
+        if (pindexBestForkTip && pindexBestForkBase)
         {
             LogPrintf("CheckForkWarningConditions: Warning: Large valid fork found\n  forking the chain at height %d (%s)\n  lasting to height %d (%s).\nChain state database corruption likely.\n",
                    pindexBestForkBase->nHeight, pindexBestForkBase->phashBlock->ToString(),