oggenc: Fix the EOS flag

This corrects the bug that caused the checksums to change in
9767d7c092c890ecc5953452e8a951fd902dd67b.

It caused the EOS flag to be set incorrectly; the ogg spec does not
allow it to be set in the middle of a logical bitstream.

Signed-off-by: Andrew Kelley <superjoe30@gmail.com>
Signed-off-by: Martin Storsjö <martin@martin.st>