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>

libavformat/avidec.c

@@ -693,6 +693,7 @@ static int avi_read_header(AVFormatContext *s)
             default:
                 av_log(s, AV_LOG_INFO, "unknown stream type %X\n", tag1);
             }
+            ast->sample_size = FFMAX(ast->sample_size, 0);
             if (ast->sample_size == 0) {
                 st->duration = st->nb_frames;
                 if (st->duration > 0 && avi->io_fsize > 0 && avi->riff_end > avi->io_fsize) {