Fix for GetBlockValue() after block 13,440,000

Forces the block reward to zero when right shift in GetBlockValue() is
undefined, after 64 reward halvings (block height 13,440,000).

src/main.cpp

@@ -1176,9 +1176,14 @@ void static PruneOrphanBlocks()
 int64_t GetBlockValue(int nHeight, int64_t nFees)
 {
     int64_t nSubsidy = 50 * COIN;
+    int halvings = nHeight / Params().SubsidyHalvingInterval();
+
+    // Force block reward to zero when right shift is undefined.
+    if (halvings >= 64)
+        return nFees;
 
     // Subsidy is cut in half every 210,000 blocks which will occur approximately every 4 years.
-    nSubsidy >>= (nHeight / Params().SubsidyHalvingInterval());
+    nSubsidy >>= halvings;
 
     return nSubsidy + nFees;
 }