fix: don't mangle memory for zombie processes

[joske]

On macOS, for a zombie process, proc_pidpath returns 0, and nothing is written in fullname, so it's uninitialized garbage. We should not try to do any string mangling on it.

There is no program name, and ps -ef returns <defunct> for the zombie, so I'm doing the same for btop.

[joske]

You can easily test this with the following little program that creates a zombie process:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

// This program creates a zombie process, the parent never calls wait()
int main() {
  int pid = fork();
  if (pid == 0) {
    exit(0);
  } else {
    printf("Child process: %d\n", pid);
    for (;;) {
      sleep(30);
    }
  }
  return 0;
}

Kill the parent to get rid of the zombie.