darwin: process' percent_cpu should be updated only if the process' r…

…unning time has changed

Fixes: #1619

darwin/DarwinProcess.c

@@ -370,13 +370,12 @@ void DarwinProcess_setFromLibprocPidinfo(DarwinProcess* proc, DarwinProcessTable
    const DarwinMachine* dhost = (const DarwinMachine*) proc->super.super.host;
 
    uint64_t total_existing_time_ns = proc->stime + proc->utime;
-
    uint64_t user_time_ns = pti.pti_total_user;
    uint64_t system_time_ns = pti.pti_total_system;
    uint64_t total_current_time_ns = user_time_ns + system_time_ns;
 
-   if (total_existing_time_ns && 1E-6 < timeIntervalNS) {
-      uint64_t total_time_diff_ns = total_current_time_ns - total_existing_time_ns;
+   if (total_existing_time_ns < total_current_time_ns) {
+      const uint64_t total_time_diff_ns = total_current_time_ns - total_existing_time_ns;
       proc->super.percent_cpu = ((double)total_time_diff_ns / timeIntervalNS) * 100.0;
    } else {
       proc->super.percent_cpu = 0.0;