ps/display.c: Harden show_tree().

ps/display.c: Harden show_tree().

1/ Do not go deeper than the size of forest_prefix[], to prevent a
buffer overflow (sizeof(forest_prefix) is roughly 128K, but the maximum
/proc/sys/kernel/pid_max is 4M). (actually, we go deeper, but we stop
adding bytes to forest_prefix[])

2/ Always null-terminate forest_prefix[] at the current level.