hw2: Potential `readline` Memory Leak

[tjhu]

credit: https://piazza.com/class/ktx3wfc7ufz711?cid=63

Leak location:
https://github.com/antonburtsev/cs143a/blob/7afb45856cc7016e0be1aadf0decc34e9e273a82/hw/hw2-shell/sh_t.c#L40-L47

Potential fix:

if (feof(stdin))  // EOF
    {
      free(line);
      fprintf(stderr, "EOF\n");
      exit(EXIT_SUCCESS);
    }

Need to verify that the leak indeed occurs and the fix fixes it.