checks_external.c correction

[testos]

I'm working for several weeks in a few external scripts for:
- restart zabbix agent in windows/linux machines from zabbix server
- stop zabbix agent in windows/linux machines from zabbix server
- start zabbix agent in windows/linux machines from zabbix server
- status zabbix agent in windows/linux machines from zabbix server
- copy a new start/stop/restart zabbix agent script in windows/linux machines from zabbix server
- remotely zabbix agent installation in windows/linux machines from zabbix server

I discovered that checks_external.c returned nothing with scripts containing instructions to creating child processes within as sleep, scp, ssh, while, and so on.
checks_external.c use popen() function for "open'' a process by creating a bidirectional pipe, forking, and invoking the shell, but it don't wait for the last child spawned by the parent script.
I patched checks_external.c adding wait(NULL); instruction just below popen and now everything works perfectly.

    if (NULL == (fp = popen(cmd, "r"))) 
    {
        zbx_snprintf(error,MAX_STRING_LEN-1,"External check [%s] is not supported, failed execution", item->key);
        zabbix_log( LOG_LEVEL_DEBUG, "%s", error);
        SET_STR_RESULT(result, strdup(error));
        return NOTSUPPORTED;
    }
    wait(NULL);
    /* we only care about the first line */