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>