avf: fix faulty check in has_duration

An invalid duration is AV_NOPTS_VALUE not 0.

libavformat/utils.c

@@ -1823,7 +1823,7 @@ static int has_duration(AVFormatContext *ic)
         if (st->duration != AV_NOPTS_VALUE)
             return 1;
     }
-    if (ic->duration)
+    if (ic->duration != AV_NOPTS_VALUE)
         return 1;
     return 0;
 }