riff: don't overwrite bps from WAVEFORMATEX if EXTENSIBLE doesn't contain that data.

According to the specification on the MSDN [1], 0 is valid for that
particular field, and it should be ignored in that case.

[1]: http://msdn.microsoft.com/en-us/library/windows/desktop/dd757714(v=vs.85).aspx

Bug-Id: 950

Signed-off-by: Anton Khirnov <anton@khirnov.net>
