qcow2: Do not overflow when writing an L1 sector

qcow2: Do not overflow when writing an L1 sector

While writing an L1 table sector, qcow2_write_l1_entry() copies the
respective range from s->l1_table to the local "buf" array. The size of
s->l1_table does not have to be a multiple of L1_ENTRIES_PER_SECTOR;
thus, limit the index which is used for copying all entries to the L1
size.

Cc: [email protected]
Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Peter Lieven <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>