grp_struct_to_py(): fix error handling

grp_struct_to_py(): fix error handling

Both getgrgid_r() and getgrnam_r() *return* an error number
on failures, and don't store it to errno. Thus, rc will not
be less than zero, and we need to set errno before we can
create a python error from it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>