LU-599 verify "mi_ginfo" before accessing its member

LU-599 verify "mi_ginfo" before accessing its member

If the target user only belong to one group, then we just set
"identity->mi_gid", and does not set "identity->mi_ginfo", to
save unnecessary memory allocation and groups processing.

So before accessing member inside "identity->mi_ginfo", make
sure "identity->mi_ginfo" is valid.

Change-Id: I880ccadddd93425c301115328d57c552d11bda8e
Signed-off-by: nasf <yong.fan@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1252
Tested-by: Hudson
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: hongchao.zhang <hongchao.zhang@whamcloud.com>
Tested-by: hongchao.zhang <hongchao.zhang@whamcloud.com>
Tested-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>