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

Given the following example configuration:

    listen foo
    	mode http
    	bind *:8080
    	http-request set-var(txn.leak) bool(1)
    	server x example.com:80

Running a configuration check with valgrind reports:

    ==24233== 2 bytes in 1 blocks are definitely lost in loss record 1 of 345
    ==24233==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==24233==    by 0x4E238D: my_strndup (tools.c:2261)
    ==24233==    by 0x581E10: make_arg_list (arg.c:253)
    ==24233==    by 0x4DE90D: sample_parse_expr (sample.c:890)
    ==24233==    by 0x58E2F4: parse_store (vars.c:772)
    ==24233==    by 0x566A2F: parse_http_req_cond (http_rules.c:95)
    ==24233==    by 0x4A4CE6: cfg_parse_listen (cfgparse-listen.c:1339)
    ==24233==    by 0x494C59: readcfgfile (cfgparse.c:2049)
    ==24233==    by 0x545135: init (haproxy.c:2029)
    ==24233==    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+.

(cherry picked from commit c7d8a86f2f8ccdcaada18a5e84bd5cb8c27dac51)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 300f65dfa5d5bdb51cd3283d92bf84793f959b86)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit eb2fc048673d071888e6d9889c5b6ece87dec09c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/sample.c b/src/sample.c
index 5bd46f8..fbc422b 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -3040,12 +3040,14 @@
 {
 	if (strcasecmp(args[0].data.str.area, "true") == 0 ||
 	    strcasecmp(args[0].data.str.area, "1") == 0) {
+		free(args[0].data.str.area);
 		args[0].type = ARGT_SINT;
 		args[0].data.sint = 1;
 		return 1;
 	}
 	if (strcasecmp(args[0].data.str.area, "false") == 0 ||
 	    strcasecmp(args[0].data.str.area, "0") == 0) {
+		free(args[0].data.str.area);
 		args[0].type = ARGT_SINT;
 		args[0].data.sint = 0;
 		return 1;