fix nil pointer dereference in amazon-chroot
fix nil pointer dereference in amazon-chroot

The refactor of StepRegisterAmi to add the from_scratch parameter
incorrectly replaced iteration over image.BlockDeviceMappings with
iteration over the new mapping array when not using from_scratch.  This
fixes the regression and renames the parameters to newMappings and
mappings for better clarity.

mitchellh/packer,smaato/packer,rnaveiras/packer,pvanbuijtene/packer,tsmolka/packer,dayglojesus/packer,pecigonzalo/packer,boumenot/packer,tb3088/packer,boumenot/packer,mkuzmin/packer,legal90/packer,grubernaut/packer,zhuzhih2017/packer,routelastresort/packer,routelastresort/packer,jen20/packer,rickard-von-essen/packer,rnaveiras/packer,jen20/packer,youhong316/packer,tsmolka/packer,pieter-lazzaro/packer,pvanbuijtene/packer,tb3088/packer,rnaveiras/packer,taliesins/packer,dave2/packer,samdunne/packer,life360/packer,pieter-lazzaro/packer,KohlsTechnology/packer,markpeek/packer,jimmythedog/packer,c22/packer,ricardclau/packer,monkeylittleinc/packer,dave2/packer,ChrisLundquist/packer,markpeek/packer,mwhooker/packer,zhuzhih2017/packer,smaato/packer,markpeek/packer,rickard-von-essen/packer,samdunne/packer,mwhooker/packer,atlassian/packer,grubernaut/packer,bryson/packer,pecigonzalo/packer,arizvisa/packer,dayglojesus/packer,taliesins/packer,mitchellh/packer,p1-nico/packer,pvanbuijtene/packer,p1-nico/packer,tas50/packer,mkuzmin/packer,timsutton/packer,sneal/packer,arizvisa/packer,jimmythedog/packer,p1-nico/packer,youhong316/packer,mwhooker/packer,mafrosis/packer,tas50/packer,mitchellh/packer,taliesins/packer,ricardclau/packer,hashicorp/packer,smaato/packer,samdunne/packer,ChrisLundquist/packer,tb3088/packer,sean-/packer,monkeylittleinc/packer,paulmey/packer,hashicorp/packer,pecigonzalo/packer,threatstream/packer,atlassian/packer,jimmythedog/packer,mitchellh/packer,life360/packer,taliesins/packer,arizvisa/packer,mwhooker/packer,tsmolka/packer,tb3088/packer,jen20/packer,ricardclau/packer,monkeylittleinc/packer,boumenot/packer,paulmey/packer,atlassian/packer,markpeek/packer,dayglojesus/packer,pvandervelde/packer,timsutton/packer,routelastresort/packer,sean-/packer,tb3088/packer,mafrosis/packer,sneal/packer,c22/packer,paulmey/packer,tas50/packer,sean-/packer,pvandervelde/packer,tsmolka/packer,tsmolka/packer,ricardclau/packer,timsutton/packer,sneal/packer,dave2/packer,pieter-lazzaro/packer,youhong316/packer,jen20/packer,jimmythedog/packer,legal90/packer,p1-nico/packer,jimmythedog/packer,dave2/packer,taliesins/packer,quantang/packer,tsmolka/packer,legal90/packer,ChrisLundquist/packer,bryson/packer,smaato/packer,arizvisa/packer,life360/packer,grubernaut/packer,pieter-lazzaro/packer,zhuzhih2017/packer,pvandervelde/packer,ricardclau/packer,life360/packer,mkuzmin/packer,hashicorp/packer,sean-/packer,sneal/packer,rnaveiras/packer,dave2/packer,paulmey/packer,mkuzmin/packer,markpeek/packer,KohlsTechnology/packer,routelastresort/packer,sean-/packer,bryson/packer,c22/packer,dave2/packer,taliesins/packer,quantang/packer,samdunne/packer,mkuzmin/packer,grubernaut/packer,quantang/packer,smaato/packer,zhuzhih2017/packer,mafrosis/packer,tb3088/packer,KohlsTechnology/packer,dayglojesus/packer,sean-/packer,youhong316/packer,pieter-lazzaro/packer,timsutton/packer,rickard-von-essen/packer,mafrosis/packer,pvandervelde/packer,rnaveiras/packer,youhong316/packer,timsutton/packer,quantang/packer,p1-nico/packer,pieter-lazzaro/packer,boumenot/packer,threatstream/packer,paulmey/packer,KohlsTechnology/packer,grubernaut/packer,monkeylittleinc/packer,routelastresort/packer,tas50/packer,c22/packer,bryson/packer,quantang/packer,smaato/packer,KohlsTechnology/packer,p1-nico/packer,zhuzhih2017/packer,pvanbuijtene/packer,KohlsTechnology/packer,ricardclau/packer,life360/packer,pecigonzalo/packer,arizvisa/packer,jen20/packer,youhong316/packer,markpeek/packer,mwhooker/packer,legal90/packer,c22/packer,threatstream/packer,atlassian/packer,pvanbuijtene/packer,mafrosis/packer,rickard-von-essen/packer,sneal/packer,jen20/packer,paulmey/packer,mkuzmin/packer,hashicorp/packer,arizvisa/packer,bryson/packer,zhuzhih2017/packer,c22/packer,pvandervelde/packer,bryson/packer,ChrisLundquist/packer,legal90/packer,samdunne/packer,atlassian/packer,rickard-von-essen/packer,monkeylittleinc/packer,timsutton/packer,grubernaut/packer,ChrisLundquist/packer,rnaveiras/packer,dayglojesus/packer,threatstream/packer,pvandervelde/packer,threatstream/packer,mitchellh/packer,tas50/packer,ChrisLundquist/packer,atlassian/packer,tas50/packer,boumenot/packer