vorbiscomment: Fix handling very short files.

When setting up a new chain link, vcedit_open_callbacks always tries to read a new chunk at the edit of the stream identification loop. This could fail if the whole segment fits within a single, already buffered chunk.

Instead, check for a new available page before trying to buffer more data. As long as there's a data page available after the header pages, this will succeed, exiting early without trying to load data unnecessarily.

Thanks to lsd for reporting the issue.