Fix snippet buffered read logic

The snippet generates a file with 215 bytes.
It reads the file with a 1024 byte buffer.
While this works with a new 0-initialized buffer, the buffered reading misses using the returned read bytes count.
A file with > 1024 bytes will reuse the now non-0 buffer and print unintended values.

Fix buffered reading to be technically correct and work for arbitrary file contents.

Fixes dotnet#4141

snippets/csharp/System.IO/FileStream/Overview/fstream class.cs

@@ -35,9 +35,10 @@ public static void Main()
         {
             byte[] b = new byte[1024];
             UTF8Encoding temp = new UTF8Encoding(true);
-            while (fs.Read(b,0,b.Length) > 0)
+            int readLen;
+            while ((readLen = fs.Read(b,0,b.Length)) > 0)
             {
-                Console.WriteLine(temp.GetString(b));
+                Console.WriteLine(temp.GetString(b,0,readLen));
             }
         }
     }