fix incorrect boolean check

https://stackoverflow.com/questions/767372/java-string-equals-versus/767379

Author: anon-sportycto

src/noobchain/Block.java

@@ -46,7 +46,7 @@ public void mineBlock(int difficulty) {
 	public boolean addTransaction(Transaction transaction) {
 		//process transaction and check if valid, unless block is genesis block then ignore.
 		if(transaction == null) return false;		
-		if((previousHash != "0")) {
+		if((!"0".equals(previousHash))) {
 			if((transaction.processTransaction() != true)) {
 				System.out.println("Transaction failed to process. Discarded.");
 				return false;