Attempted fix to Trac #1486

Attempted fix to Trac #1486

beginning-of-stream seek targets were no longer well defined in
ov_pcm_seek_page() (and thus ov_pcm_seek()) since extending to
multiplexed streams.  Beginning of stream is a special case due to the
seek target preceding the first explicit granule position.

Although seeking to PCM 0 triggered the bug, early seeks in general
were not ebing handld well; rather than continuing to overload the
bisection, handle the early-seek case outside the loop.

svn path=/trunk/vorbis/; revision=19159