userns_exec_1: catch errors in the spawned process.

lxc_map_ids can call system(3), which on error from the
spawned process returns > 0.  No path should return > 0
when it meant success.  So check the lxc_map_ids() value
to be != rather than just < 0.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>