fix getVarlenBytes implementation

Author: smoothdeveloper

src/ZMidi/Read.fs

@@ -53,7 +53,15 @@ module ReadFile =
         return (uint32 result) }
 
     let getVarlenText = gencount getVarlen readChar (fun _ b -> System.String b)
-    let getVarlenBytes = gencount getVarlen readByte (fun _ b -> b)
+    let getVarlenBytes =
+        parseMidi {
+          let! l = getVarlen
+          return! ParserMonad (
+                    fun data state ->
+                      Ok (data.[state.Position .. state.Position + (int l) - 1], { state with Position = state.Position + int l})
+                  )
+        }
+        
     let deltaTime = 
       parseMidi {
           let! v = getVarlen