block/cloop: fix offsets[] size off-by-one

cloop stores the number of compressed blocks in the n_blocks header
field.  The file actually contains n_blocks + 1 offsets, where the extra
offset is the end-of-file offset.

The following line in cloop_read_block() results in an out-of-bounds
offsets[] access:

    uint32_t bytes = s->offsets[block_num + 1] - s->offsets[block_num];

This patch allocates and loads the extra offset so that
cloop_read_block() works correctly when the last block is accessed.

Notice that we must free s->offsets[] unconditionally now since there is
always an end-of-file offset.

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>
