apparmor: fix uninitialised pointer passed to audit_log_untrustedstring()

In the Linux kernel, the following vulnerability has been resolved:

apparmor: fix uninitialised pointer passed to audit_log_untrustedstring()

Commit 4a134723f9f1 ("apparmor: move check for aa_null file to cover all cases")
intrdouced a small bug, where path_name() may pass a potentially uninitialized
*name to aa_audit_file() if the path->dentry had been replaced with
aa_null.dentry earlier on. This can lead to page fault like one observed on
7.0.2 openSUSE Tumbleweed kernel:

[51692.242756] [ T24690] BUG: unable to handle page fault for address: 0000000f00000003
[51692.242762] [ T24690] #PF: supervisor read access in kernel mode
[51692.242763] [ T24690] #PF: error_code(0x0000) - not-present page
[51692.242765] [ T24690] PGD 0 P4D 0
[51692.242768] [ T24690] Oops: Oops: 0000 [#1] SMP NOPTI
[51692.242772] [ T24690] CPU: 3 UID: 1020 PID: 24690 Comm: snap-confine Tainted: G O 7.0.2-1-default #1 PREEMPT(full) openSUSE Tumbleweed ab90b4c9940707f9cafa19bdad80b2cec52dbe51
[51692.242775] [ T24690] Tainted: [O]=OOT_MODULE
[51692.242777] [ T24690] Hardware name: Framework Laptop 13 (AMD Ryzen 7040Series)/FRANMDCP05, BIOS 03.18 01/08/2026
[51692.242778] [ T24690] RIP: 0010:strlen+0x4/0x30
[51692.242783] [ T24690] Code: f7 75 ec 31 c0 e9 17 9f 00 ff 48 89 f8 e9 0f 9f 00 ff 0f 1f 40 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa <80> 3f 00 74 18 48 89 f8 0f 1f 40 00 48 83 c0 01 80 38 00 75 f7 48
[51692.242785] [ T24690] RSP: 0018:ffffd015eb1e3608 EFLAGS: 00010282
[51692.242787] [ T24690] RAX: 0000000000000000 RBX: ffff89796198a360 RCX: 0000000000000000
[51692.242788] [ T24690] RDX: 00000000000000d1 RSI: 0000000f00000003 RDI: 0000000f00000003
[51692.242790] [ T24690] RBP: ffffffffb7ede090 R08: 00000000000005f5 R09: 0000000000000000
[51692.242791] [ T24690] R10: 0000000000000000 R11: 0000000000000000 R12: ffffd015eb1e3700
[51692.242792] [ T24690] R13: ffff8977a22bc380 R14: ffffffffb7ec5190 R15: ffff8977a0c8aa80
[51692.242794] [ T24690] FS: 0000000000000000(0000) GS:ffff897f640d8000(0000) knlGS:0000000000000000
[51692.242796] [ T24690] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[51692.242797] [ T24690] CR2: 0000000f00000003 CR3: 00000006ad15f000 CR4: 0000000000f50ef0
[51692.242799] [ T24690] PKRU: 55555554
[51692.242800] [ T24690] Call Trace:
[51692.242802] [ T24690] <TASK>
[51692.242804] [ T24690] audit_log_untrustedstring+0x1d/0x40
[51692.242811] [ T24690] common_lsm_audit+0x71/0x1d0
[51692.242816] [ T24690] aa_audit+0x5a/0x170
[51692.242819] [ T24690] aa_audit_file+0x18a/0x1b0
[51692.242825] [ T24690] path_name+0xd2/0x100
[51692.242829] [ T24690] profile_path_perm.part.0+0x58/0xb0
[51692.242832] [ T24690] aa_path_perm+0xef/0x150
[51692.242837] [ T24690] apparmor_file_open+0x153/0x2e0
[51692.242840] [ T24690] security_file_open+0x46/0xd0
[51692.242844] [ T24690] do_dentry_open+0xe9/0x4d0
[51692.242848] [ T24690] vfs_open+0x30/0x100

While here, initialise variables which are passed down to path_name().