avf: has_duration does not check the global one

Some container formats report a global duration, but not a per stream
one.

libavformat/utils.c

@@ -1823,6 +1823,8 @@ static int has_duration(AVFormatContext *ic)
         if (st->duration != AV_NOPTS_VALUE)
             return 1;
     }
+    if (ic->duration)
+        return 1;
     return 0;
 }