shifting a negative signed value is undefined ("-1L << 16" in inflate.c)

inflate.c triggers the following warning in clang version 3.8.0-2ubuntu4:

```
inflate.c:1507:61: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
    if (strm == Z_NULL || strm->state == Z_NULL) return -1L << 16;
                                                        ~~~ ^
```

I believe `~0UL << 16` has defined behavior and yields the expected result.