gmputil: add nft_gmp_free() to free strings from mpz_get_str()

gmputil: add nft_gmp_free() to free strings from mpz_get_str()

mpz_get_str() (with NULL as first argument) will allocate a buffer using
the allocator functions (mp_set_memory_functions()). We should free
those buffers with the corresponding free function.

Add nft_gmp_free() for that and use it.

The name nft_gmp_free() is chosen because "mini-gmp.c" already has an
internal define called gmp_free(). There wouldn't be a direct conflict,
but using the same name is confusing. And maybe our own defines should
have a clear nft prefix.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>