Keep next_in and avail_in in sync in bz2/lzma/zlib decompressor reset paths

[msmeissn]

Bug report

Bug description:

(This was found by Gemini, orchestrated by Dirk Mueller at SUSE)

For GHSA-pg25-7cx5-cvcm the "next_in" pointers were reset to NULL. However if the decompressor is reused, the "avail_in_real" counter should also be reset to "0" at this time, as if its reused again, next_in and avail_in_real might not be in sync again.

This affects

Modules/_bz2module.c
Modules/_lzmamodule.c
Modules/zlibmodule.c

For bz2, everywhere where next_in is set to NULL, avail_in_real also needs to be set to 0.

++ bzs->next_in = NULL;
should also set
bzs->avail_in_real = 0;

similar for the others.

This is largely hardening. It is not clear if these use cases exist.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

[StanFromIreland]

Please note that there is no bug currently, as we use it only if bzs->next_in != NULL (which we do clear). It is up to the compression module maintainers if they want to accept such a hardening.