Fix REALLOC_ZERO_BYTES_FREES comment

[Florian Weimer]

* Paul Eggert:

> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 0cd3ba78ca..e2d7b1b583 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -346,13 +346,14 @@ __malloc_assert (const char *assertion, const char *file, unsigned int line,
>  #define REVEAL_PTR(ptr)  PROTECT_PTR (&ptr, ptr)
>  
>  /*
> -  REALLOC_ZERO_BYTES_FREES controls the behavior of realloc (p, 0)
> -  when p is nonnull.  If nonzero, realloc (p, 0) should free p and
> -  return NULL.  Otherwise, realloc (p, 0) should do the equivalent
> -  of freeing p and returning what malloc (0) would return.
> -
> -  ISO C17 says the behavior is implementation-defined here; glibc
> -  follows historical practice and defines it to be nonzero.
> +  The REALLOC_ZERO_BYTES_FREES macro controls the behavior of realloc (p, 0)
> +  when p is nonnull.  If the macro is nonzero, the realloc call returns NULL;
> +  otherwise, the call returns what malloc (0) would.  In either case,
> +  p is freed.  Glibc uses a nonzero REALLOC_ZERO_BYTES_FREES, which
> +  implements common historical practice.
> +
> +  ISO C17 says the realloc call has implementation-defined behavior,
> +  and it might not even free p.
>  */

I think this is a really good explanation of the situation.  Thanks.

Florian