indexcodec: Make sure codeaux_table sentinels are zero
indexcodec: Make sure codeaux_table sentinels are zero

This will make sure we can use these as version data in the future.

In the existing stream, they can be found by looking at the last two
bytes of the encoded stream, so the version will have to be in the
footer.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer