Check type of "path" before accessing it
Check type of "path" before accessing it

This fix CC#1977
Without this commit:
>> a: func [/b] [1]
>> a/b/%
Segmentation fault

With this commit
>> a: func [/b] [1]
>> a/b/%
** Script error: incompatible refinement: %""

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