avidec: avoid infinite loop due to negative ast->sample_size

If max in clean_index is set to a negative ast->sample_size, the
following loop never ends:
        while (max < 1024)
            max += max;

Thus set ast->sample_size to 0 if it would otherwise be negative.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
