BUG/MINOR: haproxy: Free srule->expr during deinit

BUG/MINOR: haproxy: Free srule->expr during deinit

Given the following example configuration:

    backend foo
    	mode http
    	use-server %[str(x)] if { always_true }
    	server x example.com:80

Running a configuration check with valgrind reports:

    ==19376== 170 (40 direct, 130 indirect) bytes in 1 blocks are definitely lost in loss record 281 of 347
    ==19376==    at 0x4C2FB55: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==19376==    by 0x5091AC: add_sample_to_logformat_list (log.c:511)
    ==19376==    by 0x50A5A6: parse_logformat_string (log.c:671)
    ==19376==    by 0x4957F2: check_config_validity (cfgparse.c:2588)
    ==19376==    by 0x54442D: init (haproxy.c:2129)
    ==19376==    by 0x421E42: main (haproxy.c:3169)

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

This is a very minor leak that can only be observed if deinit() is called,
shortly before the OS will free all memory of the process anyway. No
backport needed.