Commit tterribe's port of Tremor r17541:

Commit tterribe's port of Tremor r17541:
Harden the code that trims the last packet of a stream; it was
possible to game the granpos such that the trim code would try to
rewind more samples than were actually available in storage.

Also, fix/eliminate two printf warnings in seeking_example extension.

svn path=/trunk/vorbis/; revision=17561