make brainwaresrcio handle cases with no spikes in a segment

There was a recent bug a colleague of mine encountered in brainwaresrcio.  I tracked it to a problem when the io class is given a segment with no spikes at all.  This commit fixes the problem.  I don't have a test file yet, I will add one in a few days.

I would like to backport this fix to the 0.3 branch, then tag a 0.3.2.1 or 0.3.3 release including this and the fix for issue #113.  I don't think basic bugfixes like these should have to wait for 0.4, but I am not sure it justifies a 0.3.3 update (but a 0.3.3 update would probably be simpler to understand).