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.

Signed-off-by: Thomas Daede <tdaede@mozilla.com>