This was originally used to cause a seek to the next block prior to
reading such that successive calls to r_core_block_read() would progress
through memory one block at a time. This was broken, though, by commit
452669d941 ("more cleanup in r_core_block_read") when when it used
`next' to directly calculate the offset rather than via a seek.
Only one call site remains that attempts to read the next block instead
of the current, and this probably was not even observable due to the
"hacky fix" added in commit 3bfa61946e ("Cleaner pvj, fix tinype load,
and honor 'ao N's").
The current of semantics of `next' appear to be broken and there is very
little dependence on it. If the original behavior should be restored
anywhere, it would be much better to add a new function, or just do the
seek explicitly, rather than parameterizing r_core_block_read() on it.
9b47635f0c