[Toybox] Figured out why ps/iotop weren't displaying -o COMM properly.

[Rob Landley]

It's because readlink0() wasn't blanking the string on failure (it returned 0
but didn't write a null to the start of the supplied buffer), so for threads it
displayed the debris in the buffer (generally a single digit) rather than
falling back to the stat[2] info.

I'm pretty sure this worked at one point and then I redid stuff in lib that
broke it. :P

Rob