Handle no chain tip available in InvalidChainFound()

Handle the case where no chain tip is available, in InvalidChainFound(). This fixes a null pointer deference when running unit tests, if the genesis block or block validation code is broken.

src/main.cpp

@@ -1196,9 +1196,11 @@ void static InvalidChainFound(CBlockIndex* pindexNew)
       pindexNew->GetBlockHash().ToString(), pindexNew->nHeight,
       log(pindexNew->nChainWork.getdouble())/log(2.0), DateTimeStrFormat("%Y-%m-%d %H:%M:%S",
       pindexNew->GetBlockTime()));
+    CBlockIndex *tip = chainActive.Tip();
+    assert (tip);
     LogPrintf("%s:  current best=%s  height=%d  log2_work=%.8g  date=%s\n", __func__,
-      chainActive.Tip()->GetBlockHash().ToString(), chainActive.Height(), log(chainActive.Tip()->nChainWork.getdouble())/log(2.0),
-      DateTimeStrFormat("%Y-%m-%d %H:%M:%S", chainActive.Tip()->GetBlockTime()));
+      tip->GetBlockHash().ToString(), chainActive.Height(), log(tip->nChainWork.getdouble())/log(2.0),
+      DateTimeStrFormat("%Y-%m-%d %H:%M:%S", tip->GetBlockTime()));
     CheckForkWarningConditions();
 }