windows: fix nil pointer reference
windows: fix nil pointer reference

stuertz/restic,arithmetric/restic,ar-jan/restic,byo/restic,fawick/restic,byo/restic,jayme-github/restic,mappu/restic,kurin/restic,fawick/restic,stuertz/restic,middelink/restic,howeyc/restic,kurin/restic,ar-jan/restic,middelink/restic,ckemper67/restic,restic/restic,restic/restic,episource/restic,jayme-github/restic,bchapuis/restic,pombredanne/restic