avcodec/rv30: put the rpr check back in init with the max vs bits bug fixed

Its usefull to know immediatly if extradata is wrong and not just when later
the decoder tries to use it.

This check was removed by: a6a2282c25abe43e352010a7c3fbc92994c0bc1c
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
