bufio: fix emptyFinalToken example to handle multiple Reads

Fixes #25909

Change-Id: I9a53a1a06aab5d1877a8e9b1b8b782d77d6027a8
Reviewed-on: https://go-review.googlesource.com/c/157758Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bufio/example_test.go

@@ -94,6 +94,9 @@ func ExampleScanner_emptyFinalToken() {
 				return i + 1, data[:i], nil
 			}
 		}
+		if !atEOF {
+			return 0, nil, nil
+		}
 		// There is one final token to be delivered, which may be the empty string.
 		// Returning bufio.ErrFinalToken here tells Scan there are no more tokens after this
 		// but does not trigger an error to be returned from Scan itself.