do not call BuildMerkleTree() unnecessarily twice

src/main.cpp

@@ -2264,7 +2264,7 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
         return state.DoS(100, error("CheckBlock() : out-of-bounds SigOpCount"));
 
     // Check merkle root
-    if (fCheckMerkleRoot && block.hashMerkleRoot != block.BuildMerkleTree())
+    if (fCheckMerkleRoot && block.hashMerkleRoot != block.vMerkleTree.back())
         return state.DoS(100, error("CheckBlock() : hashMerkleRoot mismatch"));
 
     return true;