evaluate: release key expression in error path of implicit map with unknown datatype

evaluate: release key expression in error path of implicit map with unknown datatype

Detected when running:

 # nft -f tests/shell/testcases/bogons/nft-f/mapping_with_invalid_datatype_crash
 ==382584==ERROR: LeakSanitizer: detected memory leaks

 Direct leak of 144 byte(s) in 1 object(s) allocated from:
    #0 0x7fde06ca9e8f in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x7fde062924af in xmalloc src/utils.c:31
    #2 0x7fde0629266c in xzalloc src/utils.c:70
    #3 0x7fde06167299 in expr_alloc src/expression.c:46
    #4 0x7fde0616b014 in constant_expr_alloc src/expression.c:420
    #5 0x7fde06128e43 in expr_evaluate_map src/evaluate.c:2027
    #6 0x7fde06137b06 in expr_evaluate src/evaluate.c:2891
    #7 0x7fde06132417 in expr_evaluate_relational src/evaluate.c:2497
    #8 0x7fde06137b36 in expr_evaluate src/evaluate.c:2895
    #9 0x7fde06137d5f in stmt_evaluate_expr src/evaluate.c:2914
    #10 0x7fde061524c8 in stmt_evaluate src/evaluate.c:4646
    #11 0x7fde0615c9ee in rule_evaluate src/evaluate.c:5202
    #12 0x7fde061600c7 in cmd_evaluate_add src/evaluate.c:5422

Fixes: 70054e6e1c87 ("evaluate: catch implicit map expressions without known datatype")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>