sonic: make sure num_taps * channels is not larger than frame_size

If that is the case, the loop setting predictor_state in
sonic_decode_frame causes out of bounds reads of int_samples, which has
only frame_size number of elements.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
