Fix: runas: less-than-zero comparison of an unsigned value

Fixes two defects found by Coverity related to unsigned integers being
treated as signed.

Reported by Coverity:
    CID 1461333:  Control flow issues  (NO_EFFECT)
    This less-than-zero comparison of an unsigned value is never true. "buf_size < 0UL".

    CID 1461332:  Integer handling issues  (NEGATIVE_RETURNS)
    "buf_size" is passed to a parameter that cannot be negative.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id6d4a71960f2ef34f14c05e66ef5d934b7a3e524