Trace file to c:/ if working_directory is empty

When the working directory is empty, we now write the
trace file to c:/ instead of ./

When running as service, the workingdirectory seems to empty ,so
that the trace file is written to C:/windows/system32

This has different implications and makes it difficult to watch the
trace file.

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>