dmg: prevent out-of-bounds array access on terminator

When a terminator is reached the base for offsets and sectors is stored.
The following records that are processed will use this base value.

If the first record we encounter is a terminator, then calculating the
base values would result in out-of-bounds array accesses.  Don't do
that.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
