only get extension data if it actually exists

[moussaclarke]

This fixes an issue I was having where it would throw an exception in Laravel if there was no extension data in the wav fmt chunk. Therefore this just checks if extension/cb size is 22 (i.e. not 0 - see http://www-mmsp.ece.mcgill.ca/documents/audioformats/wave/wave.html) before grabbing any extension data.