rawdec: add check on sample_rate

Prevent error condition in case sample_rate is unset or set to a negative
value. In particular, fix divide-by-zero error occurring in ffmpeg due to
sample_rate set to 0 in output_packet(), in code:

                ist->next_pts += ((int64_t)AV_TIME_BASE * ist->st->codec->frame_size) /
                    ist->st->codec->sample_rate;

Fix trac ticket #324.
