daemon-unix: Properly handle missing users or groups.

daemon-unix: Properly handle missing users or groups.

From the manpages of getgrnam_r (getpwnam_r is similar):
"If no matching group record was found, these functions return 0 and
store NULL in *result."

The code checked only against errors, but non existing users didn't set
e != 0 therefore the code could try to set arbitrary uid/gid values.

Fixes: e91b927d lib/daemon: support --user option for all OVS daemon
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>