Fix panic in GroupInfo::members

[jeremija]

I recently observed undefined behavior in one of the projects I'm working on because slice::from_raw_parts is called without a null-check in GroupInfo::members. This undefined behavior was present when iterating over the resulting slice and it would just terminate prematurely when trying to chain multiple iterators. The function is pretty strict about what kind of pointers it accepts:

data must be non-null and aligned even for zero-length slices.

This undefined behavior has become a panic in debug builds in Rust 1.78.0:

For example, slice::from_raw_parts requires an aligned non-null pointer. The following use of a purposely-misaligned pointer has undefined behavior, and while if you were unlucky it may have appeared to "work" in the past, the debug assertion can now catch it:

Cause is found in rdkafka.c. I noticed there are more uses of slice::from_raw_parts so I replaced all of them except a call to Vec::from_raw_parts which seems fine. I'd appreciate feedback!