Do not request 0-length skips; sanity-check return.

Do not request 0-length skips; sanity-check return.

I noticed that my skip callback was always being invoked with a request of
0.  This is a bit wasteful since skip callbacks commonly involve a syscall
like lseek().

Also, it seems good to error out when the skip callback is buggy, and claims
to skip more than requested.

Test Plan:

```
autoreconf -ivf && ./configure && make && make check
```

The same tests fail as before, with the same error messages. If interested,
both failure logs are here:

https://github.com/snarkmaster/libarchive/commit/00c9751cde6cc888fb844b7a1fcc0f82dbaaedb1

These are on Ubuntu 14.04.