A DDT2 entry whose slot index points past the stored prefix/suffix block
(e.g. a resume-damaged or corrupt image) caused an out-of-bounds heap
read that was silently returned as sector data. Validate the pointer and
index against the arena length before copying, returning
AARUF_ERROR_CANNOT_READ_BLOCK with an Errored status instead. Also clamp
the two legacy Mode 2 fallback copies to the actual bare-sector length.