Fixes for two bugs relating to short files in vorbisfile:  1) _get_prev_page could get stuck in an infinite loop trying to seek to < 0  2) _process_packet could return OV_EOF when there were valid packets awaiting     decode.
Fixes for two bugs relating to short files in vorbisfile:
 1) _get_prev_page could get stuck in an infinite loop trying to seek to < 0
 2) _process_packet could return OV_EOF when there were valid packets awaiting
    decode.

Remaining problem: on my test file (739 samples long), seeking_example fails
the pcm exact seeking test.

svn path=/trunk/vorbis/; revision=2350

ksophocleous/vorbis,nicolaichuk/vorbis,youfearm3/vorbis,eidy/vorbis,erikd/vorbis,ShiftMediaProject/vorbis,felipebetancur/vorbis,erikd/vorbis,ShiftMediaProject/vorbis,eidy/vorbis,eidy/vorbis,ksophocleous/vorbis,emscripten-ports/Vorbis,nicolaichuk/vorbis,emscripten-ports/Vorbis,ShiftMediaProject/vorbis,emscripten-ports/Vorbis,ksophocleous/vorbis,ksophocleous/vorbis,ShiftMediaProject/vorbis,mwgoldsmith/vorbis,mwgoldsmith/vorbis,youfearm3/vorbis,felipebetancur/vorbis,eidy/vorbis,mwgoldsmith/vorbis,felipebetancur/vorbis,felipebetancur/vorbis,youfearm3/vorbis,nicolaichuk/vorbis,ShiftMediaProject/vorbis,mwgoldsmith/vorbis,erikd/vorbis,erikd/vorbis,ShiftMediaProject/vorbis,nicolaichuk/vorbis,emscripten-ports/Vorbis,youfearm3/vorbis