Escape ASCII NUL before trying to put it in XML, because XML can't cope.

Our sholes continuous build is having XML trouble with a FormatterTest
failure that outputs ASCII NUL. Ideally, our junit test runner would escape
all non-printable ASCII as \u escapes, but this seems like a minimal change,
and it's one we already have in the equivalent jtreg code.

Change-Id: Ib9b3219c25515fb358c204d45cb5064293a85b9f
1 file changed