Fix signed comparison warning in Encode_UTF8
Fix signed comparison warning in Encode_UTF8

When pulling @meshpoint's change I'd overlooked this signed warning.
Also changed a nearby C-style cast to a cast macro.

draegtun/ren-c,kealist/ren-c,kealist/ren-c,kealist/ren-c,hostilefork/rebol,codebybrett/ren-c,rgchris/ren-c,codebybrett/ren-c,kealist/ren-c,kealist/ren-c,draegtun/ren-c,draegtun/ren-c,giuliolunati/ren-c,rgchris/ren-c,draegtun/ren-c,giuliolunati/ren-c,hostilefork/rebol,codebybrett/ren-c,hostilefork/rebol,rgchris/ren-c,rgchris/ren-c,giuliolunati/ren-c,giuliolunati/ren-c,hostilefork/rebol,draegtun/ren-c,hostilefork/rebol,kealist/ren-c,codebybrett/ren-c,hostilefork/rebol,rgchris/ren-c,giuliolunati/ren-c,draegtun/ren-c,codebybrett/ren-c,codebybrett/ren-c