Make pcm_read_seek treat negative time stamps as 0, this avoids incorrectly

seeking before data_offset and is more consistent with how the generic index
seeking code handles it.

Originally committed as revision 17964 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/raw.c

@@ -232,6 +232,7 @@ int pcm_read_seek(AVFormatContext *s,
 
     if (block_align <= 0 || byte_rate <= 0)
         return -1;
+    if (timestamp < 0) timestamp = 0;
 
     /* compute the position by aligning it to block_align */
     pos = av_rescale_rnd(timestamp * byte_rate,