BUG/MINOR: httpclient-cli: Destroy http-client context if failing to start it

When the call to httpclient_start() failed, it is the caller responsibilty
to destroy the http-client context by calling httpclient_destroy(). It is
performed at several places but it was missing in the httpclient_cli
code. So let's fix it.

This patch must be backported as far as 2.6. On 3.2 and lower, it must be
applied on http_client.c.

src/httpclient_cli.c

@@ -159,6 +159,7 @@ static int hc_cli_parse(char **args, char *payload, struct appctx *appctx, void
 
 err:
 	memprintf(&err, "Can't start the HTTP client%s.\n", err ? err : "");
+	httpclient_destroy(hc);
 	return cli_err(appctx, err);
 }