maxtmpfiles comparison.

https://github.com/lemire/externalsortinginjava/blob/master/src/main/java/com/google/code/externalsorting/ExternalSort.java#L146
Shouldn't be there like following instead of `headersize` ?
```java
                if (maxtmpfiles < 0) {
                        System.err
                                .println("maxtmpfiles should be positive");
                }
```