evaluate: release mpz type in expr_evaluate_list() error path

evaluate: release mpz type in expr_evaluate_list() error path

Detected when running:

 # nft -f tests/shell/testcases/bogons/nft-f/no_integer_basetype_crash
 ==383222==ERROR: LeakSanitizer: detected memory leaks

 Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x7fe7b54a9e8f in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x7fe7b538b9a9 in __gmp_default_allocate (/lib/x86_64-linux-gnu/libgmp.so.10+0xc9a9)

Fixes: 3671c4897003 ("evaluate: guard against NULL basetype")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>