evaluate: release existing datatype when evaluating unary expression

evaluate: release existing datatype when evaluating unary expression

Use __datatype_set() to release the existing datatype before assigning
the new one, otherwise ASAN reports the following memleak:

Direct leak of 104 byte(s) in 1 object(s) allocated from:
    #0 0x7fbc8a2b89cf in __interceptor_malloc ../../../../src/libsa
    #1 0x7fbc898c96c2 in xmalloc src/utils.c:31
    #2 0x7fbc8971a182 in datatype_clone src/datatype.c:1406
    #3 0x7fbc89737c35 in expr_evaluate_unary src/evaluate.c:1366
    #4 0x7fbc89758ae9 in expr_evaluate src/evaluate.c:3057
    #5 0x7fbc89726bd9 in byteorder_conversion src/evaluate.c:243
    #6 0x7fbc89739ff0 in expr_evaluate_bitwise src/evaluate.c:1491
    #7 0x7fbc8973b4f8 in expr_evaluate_binop src/evaluate.c:1600
    #8 0x7fbc89758b01 in expr_evaluate src/evaluate.c:3059
    #9 0x7fbc8975ae0e in stmt_evaluate_arg src/evaluate.c:3198
    #10 0x7fbc8975c51d in stmt_evaluate_payload src/evaluate.c:330

Fixes: faa6908fad60 ("evaluate: clone unary expression datatype to deal with dynamic datatype")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>