Bluetooth: host: Fix bug in device name shortening handling

Fix bug in device name shortening handling leading to memory
corruption.
This is triggered by an underflow in the length field of the shortened
name when set_data_len + 2 > set_data_len_max.

Fixes: #27693

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>