refactoring: make pindexBestInvalid internal to validation.cpp

There's no need to have this member live on CChainState since it's only used
in validation.cpp.

src/validation.cpp

@@ -129,7 +129,7 @@ CScript COINBASE_FLAGS;
 
 // Internal stuff
 namespace {
-    CBlockIndex *&pindexBestInvalid = ::ChainstateActive().pindexBestInvalid;
+    CBlockIndex* pindexBestInvalid = nullptr;
 
     CCriticalSection cs_LastBlockFile;
     std::vector<CBlockFileInfo> vinfoBlockFile;