Calling ToText will cause a panic when writing \uFFFC

[xixileng]

In the TestRichText method of the test case, add rt.WriteString("\uFFFC").

=== RUN   TestRichText
--- FAIL: TestRichText (0.00s)
panic: runtime error: index out of range [0] with length 0 [recovered]
        panic: runtime error: index out of range [0] with length 0