index: Don't commit without valid m_best_block_index

Also report an error when coinstatsindex init fails.

src/index/base.cpp

@@ -211,6 +211,11 @@ bool BaseIndex::Commit()
 bool BaseIndex::CommitInternal(CDBBatch& batch)
 {
     LOCK(cs_main);
+    // Don't commit anything if we haven't indexed any block yet
+    // (this could happen if init is interrupted).
+    if (m_best_block_index == nullptr) {
+        return false;
+    }
     GetDB().WriteBestBlock(batch, m_chainstate->m_chain.GetLocator(m_best_block_index));
     return true;
 }

src/index/coinstatsindex.cpp

@@ -360,9 +360,9 @@ bool CoinStatsIndex::Init()
     if (pindex) {
         DBVal entry;
         if (!LookUpOne(*m_db, pindex, entry)) {
-            return false;
+            return error("%s: Cannot read current %s state; index may be corrupted",
+                         __func__, GetName());
         }
-
         m_transaction_output_count = entry.transaction_output_count;
         m_bogo_size = entry.bogo_size;
         m_total_amount = entry.total_amount;