evaluate: fix check for truncation in stmt_evaluate_log_prefix()

evaluate: fix check for truncation in stmt_evaluate_log_prefix()

Otherwise, nft crashes with prefix longer than 127 bytes:

 # nft add rule x y log prefix \"eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee\"

==159385==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffed5bf4a10 at pc 0x7f3134839269 bp 0x7ffed5bf48b0 sp 0x7ffed5bf4060
WRITE of size 129 at 0x7ffed5bf4a10 thread T0
    #0 0x7f3134839268 in __interceptor_memset ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:778
    #1 0x7f3133e3074e in __mpz_export_data /tmp/nftables/src/gmputil.c:110
    #2 0x7f3133d21d3c in expr_to_string /tmp/nftables/src/expression.c:192
    #3 0x7f3133ded103 in netlink_gen_log_stmt /tmp/nftables/src/netlink_linearize.c:1148
    #4 0x7f3133df33a1 in netlink_gen_stmt /tmp/nftables/src/netlink_linearize.c:1682
    [...]

Fixes: e76bb3794018 ('src: allow for variables in the log prefix string')
Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>