Fixed bug that caused a null pointer exception

src/main/java/mattrixwv/ProjectEuler/Problems/Problem13.java

@@ -141,6 +141,7 @@ public class Problem13 extends Problem{
 	//Constructor
 	public Problem13(){
 		super("Work out the first ten digits of the sum of the one-hundred 50-digit numbers");
+		nums = new ArrayList<BigInteger>();
 		reserveVectors();
 		sum = BigInteger.ZERO;
 	}