banlist: better handling of banlist in StartNode()

- only start working on/with banlist data, if reading in the banlist from
  disk didn't fail
- as CNode::setBannedIsDirty is false (default) when reading fails, we
  don't need to explicitly set it to false to prevent writing
  banlist.dat in that case either

src/net.cpp

@@ -1909,15 +1909,16 @@ void StartNode(boost::thread_group& threadGroup, CScheduler& scheduler)
     //try to read stored banlist
     CBanDB bandb;
     banmap_t banmap;
-    if (!bandb.Read(banmap))
+    if (bandb.Read(banmap)) {
+        CNode::SetBanned(banmap); // thread save setter
+        CNode::SetBannedSetDirty(false); // no need to write down, just read data
+        CNode::SweepBanned(); // sweep out unused entries
+
+        LogPrint("net", "Loaded %d banned node ips/subnets from banlist.dat  %dms\n",
+            banmap.size(), GetTimeMillis() - nStart);
+    } else
         LogPrintf("Invalid or missing banlist.dat; recreating\n");
 
-    CNode::SetBanned(banmap); //thread save setter
-    CNode::SetBannedSetDirty(false); //no need to write down just read or nonexistent data
-    CNode::SweepBanned(); //sweap out unused entries
-
-    LogPrintf("Loaded %i addresses from peers.dat  %dms\n",
-           addrman.size(), GetTimeMillis() - nStart);
     fAddressesInitialized = true;
 
     if (semOutbound == NULL) {