XTENSA Always fallback on bad CPU and set illegal on bad decoding

[wargio]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

Fixes the wrong internal logic and enforces illegal on fail decoding.

[wargio]

failures are unrelated to this PR (see dev branch)