mssdemux: fix setting codec_data when using WaveFormatEx

mssdemux: fix setting codec_data when using WaveFormatEx

When using WaveFormatEx the actual codec private data is at the end of
the wav header structure, after the cbSize field, see for example the
notes at http://wiki.multimedia.cx/index.php?title=WAVEFORMATEX

The previous code was erroneously passing the whole WAVEFORMATEX
structure as codec_data.

The smoothstreaming specifications[1,2,3] always mention WAVEFORMATEX
and never smaller structures like WAVEFORMAT or PCMWAVEFORMAT, so the
buffer can assumed to be at least 18 bytes and always consuming the wav
header gives the (possibly empty) codec private data.

[1] http://web.archive.org/web/20120907004742/http://www.iis.net/community/files/media/smoothspecs/%5BMS-SMTH%5D.pdf
[2] http://download.microsoft.com/download/B/0/B/B0B199DB-41E6-400F-90CD-C350D0C14A53/%5BMS-SSTR%5D.pdf
[3] http://download.microsoft.com/download/9/5/E/95EF66AF-9026-4BB0-A41D-A4F81802D92C/%5BMS-SSTR%5D.pdf

https://bugzilla.gnome.org/show_bug.cgi?id=699924