FAT: fix a potential null dereference, if Disk::read returns null (eg, reading past end of disk)
FAT: fix a potential null dereference, if Disk::read returns null (eg, reading past end of disk)

jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree,jmolloy/pedigree