dpll: fix NULL pointer dereference in dpll_msg_add_pin_ref_sync()

In the Linux kernel, the following vulnerability has been resolved: dpll: fix NULL pointer dereference in dpll_msg_add_pin_ref_sync() When a dpll_pin is shared across multiple dpll_device instances and those devices are being unregistered (e.g. during driver module removal), a NULL pointer dereference can occur in dpll_msg_add_pin_ref_sync(). This happens under the following conditions: - A pin is registered with two or more dpll devices (dpll_A, dpll_B) - The pin has ref_sync pairs with other pins - During unregistration of dpll_A's pins, a ref_sync partner pin is unregistered first, removing it from dpll_A->pin_refs - But since the partner pin is still registered with dpll_B, its dpll_refs is not empty, so dpll_pin_ref_sync_pair_del() does NOT run and the partner stays in the pin's ref_sync_pins xarray - When the pin itself is then unregistered from dpll_A, the delete notification calls dpll_msg_add_pin_ref_sync() which finds the partner in ref_sync_pins, passes dpll_pin_available() (partner is still registered with dpll_B), but dpll_pin_on_dpll_priv(dpll_A, partner) returns NULL because partner was already removed from dpll_A->pin_refs - The NULL priv pointer is passed to the driver's ref_sync_get callback, which dereferences it BUG: kernel NULL pointer dereference, address: 0000000000000034 Oops: Oops: 0000 [#1] SMP NOPTI RIP: 0010:zl3073x_dpll_input_pin_ref_sync_get+0x73/0x80 [zl3073x] Call Trace: dpll_msg_add_pin_ref_sync+0xb8/0x200 dpll_cmd_pin_get_one+0x3b6/0x4b0 dpll_pin_event_send+0x72/0x140 __dpll_pin_unregister+0x5a/0x2b0 dpll_pin_unregister+0x49/0x70 Fix this by skipping ref_sync pins whose priv pointer cannot be resolved for the current dpll device.