Zero-initialize Prior_Expand list
Zero-initialize Prior_Expand list

Address Sanitizer didn't catch the uninitialized reads of this one
for some reason, but putting it under Valgrind did.

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