tools/power turbostat: Set per_cpu_msr_sum to NULL after free

Set per_cpu_msr_sum to NULL after freeing it in the error path
of msr_sum_record() to prevent potential use-after-free issues.

Signed-off-by: Emily Ehlert <ehemily@amazon.com>
Signed-off-by: Len Brown <len.brown@intel.com>

tools/power/x86/turbostat/turbostat.c

@@ -6652,6 +6652,7 @@ release_timer:
 	timer_delete(timerid);
 release_msr:
 	free(per_cpu_msr_sum);
+	per_cpu_msr_sum = NULL;
 }
 
 /*