Avoid dereferencing interruption_point if it is nullptr

Author: MarcoFalke

src/kernel/coinstats.cpp

@@ -123,7 +123,7 @@ static bool ComputeUTXOStats(CCoinsView* view, CCoinsStats& stats, T hash_obj, c
     uint256 prevkey;
     std::map<uint32_t, Coin> outputs;
     while (pcursor->Valid()) {
-        interruption_point();
+        if (interruption_point) interruption_point();
         COutPoint key;
         Coin coin;
         if (pcursor->GetKey(key) && pcursor->GetValue(coin)) {