Fix encoder status output for modern bitrates

When the Ogg layer returns pages without any packets that end on
 them (e.g., for packets larger than ~64 kB), they don't have a
 valid timestamp.
Thus our attempts to print bitrate and progress messages were
 outputing garbage.
Instead, simply don't update the messages in this case.

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>