src/wav.c : Ensure all non-PCM files contain a 'fact' chunk.

According to:

    http://www-mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/WAVE.html

all non-PCM formats must have a 'fact' chunk. This commit does two things:

* Add code to write a warning to the internal log file on file read/open if
  there is no 'fact' chunks.
* Add a 'fact' chunk on write as needed.

See: https://github.com/erikd/libsndfile/issues/70
src/wav.c : Fix length of 'data' section.

The WAV file 'data' chunk should always be an even number of bytes in
length. To ensure this we add a padding byte as needed. On read we
check the 'data' chunk length and write a warning to the internal log
file if the 'data' chunk is not an even number of bytes in length.

Closes: https://github.com/erikd/libsndfile/issues/61