It is believed to be harmless, see a similar change 077a890a76ff.

Reported-by: geeknik <geeknik@protonmail.ch>

For instance, the last_buf flag is used in the http proxy module when
creating HTTP/2 requests to indicate the output is closed.  The flag
is inherited in ngx_output_chain() to a destination buffer when reading
the buffered request body.  Then it is used in the proxy output filter
to mark the last HTTP/2 DATA frame with END_STREAM.

The problem happens when reusing the destination buffer, such as to
re-read the buffered request body on next upstream, because this buffer
may contain a dirty last_buf value, which breaks sending HTTP/2 request
body in multiple output filter calls.

The flush and last_in_chain flags are cleared for consistency.

The XCLIENT command uses xtext encoding for attribute values,
as specified in https://www.postfix.org/XCLIENT_README.html.

Reported by Igor Morgenstern of Aisle Research.

There were a few random places where 0 was being used as a null pointer
constant.

We have a NULL macro for this very purpose, use it.

There is also some interest in actually deprecating the use of 0 as a
null pointer constant in C.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>