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.

libavformat/rawdec.c

@@ -56,6 +56,12 @@ int ff_raw_read_header(AVFormatContext *s, AVFormatParameters *ap)
 
             if (s1 && s1->sample_rate)
                 st->codec->sample_rate = s1->sample_rate;
+            if (st->codec->sample_rate <= 0) {
+                av_log(s, AV_LOG_ERROR, "Invalid sample rate %d specified\n",
+                       st->codec->sample_rate);
+                return AVERROR(EINVAL);
+            }
+
             if (s1 && s1->channels)
                 st->codec->channels    = s1->channels;