char *copy(char *dest, const char *src, int capacity) {
    for(int i = 0; i < capacity-1; i++){
        if (src[i] != NULL){
            memset(dest[i], src[i], 1);
        }
        else{
            memset(dest[i], '\0', 1);
        }
    }
    memset(dest[capacity-1], '\0', 1);
    return dest;
}
Write the copy function to perform exactly as strncpy does, with one
   exception: your copy function will guarantee that dest is always
   null-terminated. Capacity is expected to be the number of bytes of 
   memory allocated to dest.
   You shoud read the man page to learn how strncpy works.

  NOTE: You must write this function without using any string functions.
  The only function that should depend on string.h is memset.