BUG/MINOR: sample: Fix freeing of conv_exprs in release_sample_expr

BUG/MINOR: sample: Fix freeing of conv_exprs in release_sample_expr

Instead of just calling release_sample_arg(conv_expr->arg_p) we also must
free() the conv_expr itself (after removing it from the list).

Given the following example configuration:

    frontend foo
    	bind *:8080
    	mode http
    	http-request set-var(txn.foo) str(bar)
    	acl is_match str(foo),strcmp(txn.hash) -m bool

Running a configuration check within valgrind reports:

    ==1431== 32 bytes in 1 blocks are definitely lost in loss record 20 of 43
    ==1431==    at 0x4C2FB55: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==1431==    by 0x4C39B5: sample_parse_expr (sample.c:982)
    ==1431==    by 0x56B410: parse_acl_expr (acl.c:319)
    ==1431==    by 0x56BA7F: parse_acl (acl.c:697)
    ==1431==    by 0x48D225: cfg_parse_listen (cfgparse-listen.c:816)
    ==1431==    by 0x4797C3: readcfgfile (cfgparse.c:2167)
    ==1431==    by 0x52943D: init (haproxy.c:2021)
    ==1431==    by 0x41F382: main (haproxy.c:3133)

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

This is a fairly minor leak that can only be observed if samples need to be
freed, which is not something that should occur during normal processing and
most likely only during shut down. Thus no backport should be needed.