runner: Fix path handling

absolute_path() tends to return NULL if more than the last element of
the path is nonexistent. That behavior is confusing the callers, which
use NULL as a convention for something not being set at all.

Let's fix that by sprinkling a little bit of recursion onto
absolute_path() and wrapping POSIX in some sanity.

Cc: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>