Fix foreach with set-word
Fix foreach with set-word

Without this commit, this causes a busy loop:

>> foreach [a:] [1 2 3][ print mold a]

Fix CC#1751

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