rkt/image: clean asc object before every fetch
rkt/image: clean asc object before every fetch

If we don't do this, we could use the asc object from a previous image,
resulting on the incorrect signature being fetched.

Fixes https://github.com/coreos/rkt/issues/1982

yifan-gu/rkt,monstermunchkin/rkt,s-urbaniak/rkt,tmrts/rkt,markllama/rkt,Shashwatsh/rkt,coreos/rkt,achanda/rkt,jonboulle/rkt,jakon89/rkt,kinvolk/rkt,jonboulle/rkt,rkt/rkt,yifan-gu/rocket,sgotti/rkt,joshix/rkt,squeed/rkt,linki/rkt,euank/rkt,kinvolk/rkt,sinfomicien/rkt,coreos/rkt,Wilybald/rkt,jakon89/rkt,Wilybald/rkt,alban/rkt,jjlakis/rkt,shobhit85/rkt,monstermunchkin/rkt,joshix/rkt,nhlfr/rkt,cab105/rkt,intelsdi-x/rkt,sinfomicien/rkt,matthaias/rkt,kbrwn/rkt,yifan-gu/rkt,endocode/rkt,kbrwn/rkt,mike1729/rkt,ybubnov/rkt,lucasallan/rkt,kbrwn/rkt,joshix/rkt,nhlfr/rkt,jjlakis/rkt,il9ue/rkt,s-urbaniak/rkt,mike1729/rkt,dgonyeo/rkt,matthaias/rkt,Wilybald/rkt,joshix/rkt,dgonyeo/rkt,yifan-gu/rkt,euank/rkt,linki/rkt,nhlfr/rkt,ybubnov/rkt,kinvolk/rkt,akshaykarle/rkt,linki/rkt,lucasallan/rkt,akshaykarle/rkt,akshaykarle/rkt,squeed/rkt,sgotti/rkt,jonboulle/rkt,markllama/rkt,lucasallan/rkt,markllama/rkt,rkt/rkt,coreos/rkt,shobhit85/rkt,squeed/rkt,rkt/rkt,blixtra/rkt,alban/rkt,blixtra/rkt,il9ue/rkt,jjlakis/rkt,ybubnov/rkt,endocode/rkt,il9ue/rkt,kinvolk/rkt,Shashwatsh/rkt,sgotti/rkt,euank/rkt,sstabellini/rkt,alban/rkt,dgonyeo/rkt,sstabellini/rkt,mike1729/rkt,shobhit85/rkt,achanda/rkt,lucasallan/rkt,matthaias/rkt,achanda/rkt,endocode/rkt,sinfomicien/rkt,tmrts/rkt,cab105/rkt,blixtra/rkt,monstermunchkin/rkt,yifan-gu/rocket,Shashwatsh/rkt,jakon89/rkt,rkt/rkt,tmrts/rkt,Wilybald/rkt,s-urbaniak/rkt,yifan-gu/rocket,intelsdi-x/rkt,cab105/rkt,sstabellini/rkt,intelsdi-x/rkt