add Chainstate::HasCoinsViews()

Used in subsequent commits. Also cleans up asserts in
coins_views-related convenience methods to be more exact.

src/validation.h

@@ -553,15 +553,15 @@ public:
     CCoinsViewCache& CoinsTip() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
     {
         AssertLockHeld(::cs_main);
-        assert(m_coins_views->m_cacheview);
-        return *m_coins_views->m_cacheview.get();
+        Assert(m_coins_views);
+        return *Assert(m_coins_views->m_cacheview);
     }
 
     //! @returns A reference to the on-disk UTXO set database.
     CCoinsViewDB& CoinsDB() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
     {
         AssertLockHeld(::cs_main);
-        return m_coins_views->m_dbview;
+        return Assert(m_coins_views)->m_dbview;
     }
 
     //! @returns A pointer to the mempool.
@@ -575,12 +575,15 @@ public:
     CCoinsViewErrorCatcher& CoinsErrorCatcher() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
     {
         AssertLockHeld(::cs_main);
-        return m_coins_views->m_catcherview;
+        return Assert(m_coins_views)->m_catcherview;
     }
 
     //! Destructs all objects related to accessing the UTXO set.
     void ResetCoinsViews() { m_coins_views.reset(); }
 
+    //! Does this chainstate have a UTXO set attached?
+    bool HasCoinsViews() const { return (bool)m_coins_views; }
+
     //! The cache size of the on-disk coins view.
     size_t m_coinsdb_cache_size_bytes{0};