Serial.println(c); //It returns this : md st n Þ¾ïþïK[03]À¨
You need to print out the individual characters as hex values. Printing c as a C-string will only print to the first 0 byte and so may hide some of the content. (Won't solve your problem though.)
Edit: And how is send_command_to_serial(c) implemented? Does it stop at a 0 byte too?