xmllite: Avoid warning in test_read_state related to -1 not being in XmlReadState.

[Gerald Pfeifer]

On Sun, 5 Nov 2017, Nikolay Sivov wrote:
> I can reproduce with clang 3.8.1 on Debian, just sent a fix that does 
> not use casting. Thanks for reporting.

Ah, cool.  Thanks, Nikolay!

Looking at a build log with clang, I noticed a number of further
issues and will try to submit some changes as time allows.

Gerald