Make frame slice be really at most. (#4104)

src/core/stream/frame.ml

@@ -80,7 +80,7 @@ let create ~length content_type =
 
 let content_type = Fields.map Content.format
 let sub frame ofs len = Fields.map (fun c -> Content.sub c ofs len) frame
-let slice frame len = sub frame 0 len
+let slice frame len = sub frame 0 (min len (position frame))
 let after frame offset = sub frame offset (position frame - offset)
 
 let append f f' =

tests/core/frame_test.ml

@@ -4,3 +4,7 @@ let () =
       (Frame.Fields.make ~audio:(Format_type.audio ()) ())
   in
   Typing.(pcm_t <: Lang.univ_t ())
+
+let () =
+  let f = Frame.create ~length:10 (Frame.Fields.from_list []) in
+  assert (Frame.position (Frame.slice f 20) = 10)