BUG/MINOR: sample: Free str.area in smp_check_const_meth

BUG/MINOR: sample: Free str.area in smp_check_const_meth

Given the following example configuration:

    listen foo
    	mode http
    	bind *:8080
    	http-request set-var(txn.leak) meth(GET)
    	server x example.com:80

Running a configuration check with valgrind reports:

    ==25992== 4 bytes in 1 blocks are definitely lost in loss record 1 of 344
    ==25992==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==25992==    by 0x4E239D: my_strndup (tools.c:2261)
    ==25992==    by 0x581E20: make_arg_list (arg.c:253)
    ==25992==    by 0x4DE91D: sample_parse_expr (sample.c:890)
    ==25992==    by 0x58E304: parse_store (vars.c:772)
    ==25992==    by 0x566A3F: parse_http_req_cond (http_rules.c:95)
    ==25992==    by 0x4A4CE6: cfg_parse_listen (cfgparse-listen.c:1339)
    ==25992==    by 0x494C59: readcfgfile (cfgparse.c:2049)
    ==25992==    by 0x545145: init (haproxy.c:2029)
    ==25992==    by 0x421E42: main (haproxy.c:3175)

After this patch is applied the leak is gone as expected.

This is a fairly minor leak, but it can add up for many uses of the `bool()`
sample fetch. The bug most likely exists since the `bool()` sample fetch was
introduced in commit cc103299c75c530ab3637a1698306145bdc85552. The fix may
be backported to HAProxy 1.6+.