check for bad mac command

fix #485

Author: sualko

src/lmic/lmic.c

@@ -884,7 +884,7 @@ scan_mac_cmds(
 
         /* compute length, and exit for illegal commands */
         int const cmdlen = getMacCmdSize(cmd);
-        if (cmdlen > olen - oidx) {
+        if (cmdlen == 0 || cmdlen > olen - oidx) {
             // "the first unknown command terminates processing"
             olen = oidx;
             break;