charts: fix ancestor siblings check segfault

This loop assumed ancestor->parent was !NULL, and that's not necessarily
always true.  Due to these circular linked lists from the kernel's
list.h, they're not simply NULL delimited and we need the pointer to the
actual head to detect the end of the list.  In libvmon, the head for the
siblings list is either the parent proc's children member, or the
processes member of the vmon struct.  It may be more elegant to switch
to always having a root proc in libvmon, even if it's just synthetic,
for simplifying this crap.  But for now, just determine which head is
relevant and check against it for loop termination.

Under some heavy parallel kernel compilations I was seeing occasional
vwm segfaults, and the addr2line of the ip in dmesg mapped to this
particular loop.  I'm assuming the ancestor walk landed on a top-level
process and then this sibling detection tried dereferencing the
top-level proc's NULL parent and boom, segfault.

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