Fix broken cinput

[giovannipozzobon]

Checklist

Thanks a lot for your contribution!
Please tick off the following:

• Tests run successfully (i.e. make test)
  • using the CC65 compiler
  • using the LLVM/Clang compiler
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[mlund]

Closes #65.

[mlund]

This looks good to me. The main motivation for not including the range-guard in cputsxy() is to avoid needless check on known non-zero strings. I am here assuming that most calls to cputsxy() are with non-empty strings.