Fix skipping logic for multiplexed non-Opus pages.

This bug appears to have been present since the original code
 import.
This was a "clever" rearrangement of the control flow from the
 _fetch_and_process_packet() in vorbisfile to use a do ... while(0)
 instead of a "while(1)".
However, this also makes "continue" equivalent to "break": it does
 not actually go back to the top of the loop, because the loop
 condition is false.

This bug was harmless, because ogg_stream_pagein() then refuses to
 ingest a page with the wrong serialno, but we can simplify things
 by fixing it.
The "not strictly necessary" loop is now completely unnecessary.
The extra checks that existed in vorbisfile have all been moved to
 later in the main loop, so we can just continue that one directly,
 with no wasted work, instead of embedding a smaller loop inside.

Fixes Coverity CID 149875.