Another potential issue for integer overflow #40

Hi,

Our tool found the following suspicious code in:
src/org/swiftp/CmdNLST.java

In Line 35:
public final static long MS_IN_SIX_MONTHS = 6 * 30 * 24 * 60 * 60 * 1000;

Could this be as follow to avoid integer overflow?:

public final static long MS_IN_SIX_MONTHS = 6 * 30 * 24 * 60 * 60 * 1000L;

This might be a trivial thing but I wanted to report just in case.

Thanks!