vvfat: fix out of bounds array_get usage

When reading the address of the first free entry, you cannot
use array_get without first marking all entries as occupied.

This is visible if you change the sectors per cluster on a
floppy from 2 to 1.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
