has_client.cc: fix possible null pointer dereference in `Disconnect`

device could be null (not connected to profile)

Bug: 373981915
Flag: com.android.bluetooth.flags.hap_connect_only_requested_device
Test: atest --host --no-bazel-mode bluetooth_has_test
Change-Id: I2a48e58616b242252f41c1b5622f99b3ecf295c8