Fix Use of uninitialized value in length warning

Test::Harness::Switches defined like this:
*switches = *Switches;

so after setting $switches to undef $Switches is undef too,
thus we get a warning.

However I am not sure why most of test reports dont contain
this warning. I found just one http://www.cpantesters.org/cpan/report/3f0611b6-c49a-11e2-9efe-d4dbd4c8518a

also happends on my box everytime I build something.