This function has a bug:

   char *mildlyuseful(char *a,char *b)
   {
     int n;
     char *c;
     n = strlen(a) + strlen(b);
     c = malloc(n);
     if (!c) return 0;
     while (*a) *c++ = *a++;
     while (*b) *c++ = *b++;
     *c = 0;
     return c - n;
   }

Explain what the function was probably meant to do,
give a small example showing that it does not work correctly,
and explain how to make it work correctly.