Vorbis-in-Ogg: Do not set timebase to invalid values

Avoids an assert when the sample rate is invalid and the timebase
is thus set to e.g. 1/0.
Sample file is http://samples.mplayerhq.hu/ogg/fuzzed-srate-crash.ogg

This is a quick fix for a crash, not a final solution.

Signed-off-by: Mans Rullgard <mans@mansr.com>
