Copy byte* to char array goes wrong

Question

How does the copy of char array work? When I copy the byte array to a char array something went wrong. I have the following code:

void onIncomingMQTT_CMD(char* topic, byte* payload, unsigned int length) {
   char a[length];
   int i;
   String s = "";
   for(i=0; i<length; i++) {
      a[i] = (char)payload[i];
      s = s + (char)payload[i];
   }
   //message_buff[i] = '\0';
   logToMQTT(s + "[payload]");
   a[i] = '\0';
   logToMQTT(a);


   char b[length];
   int j;
   String d = "";
   for(j=0; j<length; j++) {
      b[j] = (char)payload[j];
      d = d + (char)payload[j];
   }
   //message_buff[j] = '\0';
   logToMQTT(d + "[payload2]");
   b[j] = '\0';
   logToMQTT(b);

I expected that the array a and b would be the same. But in the log output I see that the first [payload] is correct. But the second [payload2] is different. It show chars from the topic.

ON[payload]
we[payload]

What is wrong with the copy from byte array to char array?