bluez: fix CVE-2021-3588

The cli_feat_read_cb() function in src/gatt-database.c does not perform
bounds checks on the 'offset' variable before using it as an index into
an array for reading

https://nvd.nist.gov/vuln/detail/CVE-2021-3588



Signed-off-by: Steve Sakoman <steve@sakoman.com>