perf metrics: Fix SEGV with --for-each-cgroup

perf metrics: Fix SEGV with --for-each-cgroup

Ensure the metric threshold is copied correctly or else a use of
uninitialized memory happens.

Fixes: d0a3052f6faefffc ("perf metric: Compute and print threshold values")
Reported-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230505204119.3443491-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>