Fix blank logic.

Author: toots

src/core/sources/blank.ml

@@ -40,7 +40,15 @@ class blank duration =
     method abort_track = remaining <- 0
 
     method generate_frame =
-      let length = Lazy.force Frame.size in
+      let size = Lazy.force Frame.size in
+      let length =
+        match remaining with
+          | -1 -> size
+          | r ->
+              let l = if r < size then r else size in
+              remaining <- r - l;
+              l
+      in
       let frame = Frame.create ~length self#content_type in
       let audio_len = Frame.audio_of_main length in
       let video_len = Frame.video_of_main length in