Bluetooth: hci_sync: Fix UAF on hci_abort_conn_sync

Connections may be cleanup while waiting for the commands to complete so
this attempts to check if the connection handle remains valid in case of
errors that would lead to call hci_conn_failed:

BUG: KASAN: slab-use-after-free in hci_conn_failed+0x1f/0x160
Read of size 8 at addr ffff888001376958 by task kworker/u3:0/52

CPU: 0 PID: 52 Comm: kworker/u3:0 Not tainted
6.5.0-rc1-00527-g2dfe76d58d3a #5615
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
1.16.2-1.fc38 04/01/2014
Workqueue: hci0 hci_cmd_sync_work
Call Trace:
 <TASK>
 dump_stack_lvl+0x1d/0x70
 print_report+0xce/0x620
 ? __virt_addr_valid+0xd4/0x150
 ? hci_conn_failed+0x1f/0x160
 kasan_report+0xd1/0x100
 ? hci_conn_failed+0x1f/0x160
 hci_conn_failed+0x1f/0x160
 hci_abort_conn_sync+0x237/0x360

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>