kunit: string-stream: Fix a UAF bug in kunit_init_suite()

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

kunit: string-stream: Fix a UAF bug in kunit_init_suite()

In kunit_debugfs_create_suite(), if alloc_string_stream() fails in the
kunit_suite_for_each_test_case() loop, the "suite->log = stream"
has assigned before, and the error path only free the suite->log's stream
memory but not set it to NULL, so the later string_stream_clear() of
suite->log in kunit_init_suite() will cause below UAF bug.

Set stream pointer to NULL after free to fix it.

Unable to handle kernel paging request at virtual address 006440150000030d
Mem abort info:
ESR = 0x0000000096000004
EC = 0x25: DABT (current EL), IL = 32 bits
SET = 0, FnV = 0
EA = 0, S1PTW = 0
FSC = 0x04: level 0 translation fault
Data abort info:
ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
CM = 0, WnR = 0, TnD = 0, TagAccess = 0
GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[006440150000030d] address between user and kernel address ranges
Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
Dumping ftrace buffer:
(ftrace buffer empty)
Modules linked in: iio_test_gts industrialio_gts_helper cfg80211 rfkill ipv6 [last unloaded: iio_test_gts]
CPU: 5 UID: 0 PID: 6253 Comm: modprobe Tainted: G B W N 6.12.0-rc4+ #458
Tainted: [B]=BAD_PAGE, [W]=WARN, [N]=TEST
Hardware name: linux,dummy-virt (DT)
pstate: 40000005 (nZcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : string_stream_clear+0x54/0x1ac
lr : string_stream_clear+0x1a8/0x1ac
sp : ffffffc080b47410
x29: ffffffc080b47410 x28: 006440550000030d x27: ffffff80c96b5e98
x26: ffffff80c96b5e80 x25: ffffffe461b3f6c0 x24: 0000000000000003
x23: ffffff80c96b5e88 x22: 1ffffff019cdf4fc x21: dfffffc000000000
x20: ffffff80ce6fa7e0 x19: 032202a80000186d x18: 0000000000001840
x17: 0000000000000000 x16: 0000000000000000 x15: ffffffe45c355cb4
x14: ffffffe45c35589c x13: ffffffe45c03da78 x12: ffffffb810168e75
x11: 1ffffff810168e74 x10: ffffffb810168e74 x9 : dfffffc000000000
x8 : 0000000000000004 x7 : 0000000000000003 x6 : 0000000000000001
x5 : ffffffc080b473a0 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000001 x1 : ffffffe462fbf620 x0 : dfffffc000000000
Call trace:
string_stream_clear+0x54/0x1ac
__kunit_test_suites_init+0x108/0x1d8
kunit_exec_run_tests+0xb8/0x100
kunit_module_notify+0x400/0x55c
notifier_call_chain+0xfc/0x3b4
blocking_notifier_call_chain+0x68/0x9c
do_init_module+0x24c/0x5c8
load_module+0x4acc/0x4e90
init_module_from_file+0xd4/0x128
idempotent_init_module+0x2d4/0x57c
__arm64_sys_finit_module+0xac/0x100
invoke_syscall+0x6c/0x258
el0_svc_common.constprop.0+0x160/0x22c
do_el0_svc+0x44/0x5c
el0_svc+0x48/0xb8
el0t_64_sync_handler+0x13c/0x158
el0t_64_sync+0x190/0x194
Code: f9400753 d2dff800 f2fbffe0 d343fe7c (38e06b80)
---[ end trace 0000000000000000 ]---
Kernel panic - not syncing: Oops: Fatal exception