array: Don't use realloc() with zero size in array_compress()

array: Don't use realloc() with zero size in array_compress()

The behavior of realloc(3) with zero size was apparently implementation
defined.  While glibc documents the behavior as equivalent to free(3),
that might not apply to other C libraries.  With C17, this behavior has
been deprecated, and with C23, the behavior is now undefined.  It's also
why valgrind warns about this use.

Hence, when array_compress() would call realloc() with a zero size, we
now call free() explicitly and set the pointer to NULL.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>