Duration and samples throw error on WAVE file with no sub-chunks

[joshaidan]

I noticed when using WaveInfo on WAVE valid wave files with no data (or sub-chunks) that calling duration or samples would throw the following error:

     NoMethodError:
       undefined method `/' for nil:NilClass

I've created two tests to catch this error, and submitted a possible fix for the problem. Instead of throwing an error, my proposed changes will return zero instead. Also, I added a few checks to prevent divide by zero.

[njh]

Looks great, thanks!