Don't run off the end of the array-of-freelists

Don't run off the end of the array-of-freelists

This is a fix for bug 8844, where eugenis correctly notes that there's
a sentinel value at the end of the list-of-freelists that's never
actually checked.  It's a bug since the first version of the chunked
buffer code back in 0.2.0.16-alpha.

This would probably be a crash bug if it ever happens, but nobody's
ever reported something like this, so I'm unsure whether it can occur.
It would require write_to_buf, write_to_buf_zlib, read_to_buf, or
read_to_buf_tls to get an input size of more than 32K.  Still, it's a
good idea to fix this kind of thing!