wavparse: Fix clipping of size to the file size

wavparse: Fix clipping of size to the file size

The size does not include the 8 bytes tag and length, so an additional 8 bytes
must be removed here. 8 bytes are always available at this point because
otherwise the parsing of the tag and length right above would've failed.

Thanks to Antonio Morales for finding and reporting the issue.

Fixes GHSL-2024-260
Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3888

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8054>