flac: set vendor_string.length = 0
flac: set vendor_string.length = 0

Avoids debug crash with libFLAC 1.2.1; tested on Windows (x64)

Reported-by: Денис Кулаков <1ead2a080fae468b63a183a81194eb83abc0eb88@mail.ru>

kfish/libfishsound,kfish/libfishsound