perf metrics: Avoid segv if default metricgroup isn't set

stable inclusion
from stable-v6.6.7
commit 114d0e5b99376a489067d1fa3e23167f6e6cfb12
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8SSQ4

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=114d0e5b99376a489067d1fa3e23167f6e6cfb12



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

commit e2b005d6ec0e738df584190e21d2c7ada37266a0 upstream.

A metric is default by having "Default" within its groups. The default
metricgroup name needn't be set and this can result in segv in
default_metricgroup_cmp and perf_stat__print_shadow_stats_metricgroup
that assume it has a value when there is a Default metric group. To
avoid the segv initialize the value to "".

Fixes: 1c0e4795 ("perf metrics: Sort the Default metricgroup")
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-and-tested-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Cc: James Clark <james.clark@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: John Garry <john.g.garry@oracle.com>
Cc: stable@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20231204182330.654255-1-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>