Fix possible signed integer overflow

[stoeckmann]

The block_get function could return -1 if end of block is reached.

Do not left shift a negative value to prevent undefined behavior due to signed integer overflow.

The issue has been introduced with 4a5e3e7d255f3f8eba9ecdb8bd8080db43bf0aeb which is not contained in any release.

[stoeckmann]

Adjusted to just check just high. Probably not that timing relevant, but:

• if high is -1, low is as well
• if low is -1, high will be, too

[madler]

Thanks. Fixed. I changed the (long) to (unsigned long).