Avoid file-in-use-by-another-process IO exception

The data-received event on the Process-object is sometimes called with a null for the Data-member, but evidently on a separate thread. On Windows this sometimes causes the file to be in use when the "main" ? thread tries to write to the file with actual data.

First check to see if there is data to write before accessing the file.

See http://phpbb-entelect100k.rhcloud.com/viewtopic.php?f=6&t=365