powerpc/imc-pmu: Add a null pointer check in update_events_in_group()

mainline inclusion
from mainline-v6.8-rc1
commit 0a233867a39078ebb0f575e2948593bbff5826b3
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9Q9J4
CVE: CVE-2023-52675

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0a233867a39078ebb0f575e2948593bbff5826b3



--------------------------------

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.

Fixes: 885dcd70 ("powerpc/perf: Add nest IMC PMU support")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231126093719.1440305-1-chentao@kylinos.cn


Signed-off-by: Luo Gengkun <luogengkun2@huawei.com>