bug#3898: 23.1.50; nxml-extend-after-change-region should not return t

[Chong Yidong]

> When an error occurs in nxml-extend-after-change-region it returns t
> which makes font-lock-extend-jit-lock-region-after-change to fail
> since it expects a cons with two numbers or nil.
>
> The patch below make the return value nil instead.

Thanks; applied.