Fix CheckBlockIndex for reindex.

Some tests in CheckBlockIndex require chainActive.Tip(), but when reindexing, chainActive has not been set on the first call to CheckBlockIndex.

reindex.py starts a node, mines 3 blocks, stops, and reindexes with CheckBlockIndex enabled.

src/main.cpp

@@ -3227,6 +3227,14 @@ void static CheckBlockIndex()
 
     LOCK(cs_main);
 
+    // During a reindex, we read the genesis block and call CheckBlockIndex before ActivateBestChain,
+    // so we have the genesis block in mapBlockIndex but no active chain.  (A few of the tests when
+    // iterating the block tree require that chainActive has been initialized.)
+    if (chainActive.Height() < 0) {
+        assert(mapBlockIndex.size() <= 1);
+        return;
+    }
+
     // Build forward-pointing map of the entire block tree.
     std::multimap<CBlockIndex*,CBlockIndex*> forward;
     for (BlockMap::iterator it = mapBlockIndex.begin(); it != mapBlockIndex.end(); it++) {