[gzip] Use exact type for `ft_gzip_alloc` and `ft_gzip_free`.

While a function pointer may be cast to another function pointer
type, it is required to cast the function pointer back to the
original function pointer type before calling it.  If a parameter is
a pointer the exact pointer type is required.  Using a pointer to a
different underlying type is technically undefined behavior.  The
wrapper functions `ft_gzip_alloc` and `ft_gzip_free` took
`FT_Memory` (a `FT_MemoryRec_*`) instead of `voidpf` (`void*`), so
when gzip calls these callbacks through `alloc_func` or `free_func`
it invokes undefined behavior.  On most platforms this works out as
expected, but newer undefined behavior detectors and targets like
wasm can detect this and will produce an error.

* src/gzip/ftgzip.c (ft_gzip_alloc, ft_gzip_free): Update signatures
to exactly match `alloc_func` and `free_func`, respectively.
Internally, cast the `void*` opaque pointer to `FT_Memory`.
2 files changed