do not truncate pts/dts if value is AV_NOPTS_VALUE, patch by Andy Parkins, andrew parkins at 360visiontechnology com

Originally committed as revision 7234 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -2288,8 +2288,12 @@ static void truncate_ts(AVStream *st, AVPacket *pkt){
 //    if(pkt->dts < 0)
 //        pkt->dts= 0;  //this happens for low_delay=0 and b frames, FIXME, needs further invstigation about what we should do here
 
-    pkt->pts &= pts_mask;
-    pkt->dts &= pts_mask;
+    if( pkt->pts != AV_NOPTS_VALUE ) {
+        pkt->pts &= pts_mask;
+    }
+    if( pkt->dts != AV_NOPTS_VALUE ) {
+        pkt->dts &= pts_mask;
+    }
 }
 
 /**