library: fix a potential error relating to 'exe' logic

library: fix a potential error relating to 'exe' logic

After 'errno' management was standardized, a couple of
fields were added to the <pids> api. Only 1 (PIDS_EXE)
involved dynamic memory and, unfortunately, it did not
conform to that expected normalized ENOMEM convention.

Reference(s):
. Jun 2018, added 'exe' to library
commit ad4269f1189d5a7d68765e291bcfa981b6731c25
. Nov 2017, standardized 'errno' management
commit 06be33b43e5ff3f2658e77ef79441ac2e970cfd7

Signed-off-by: Jim Warner <james.warner@comcast.net>