Fix NULL pointer dereference

Fix NULL pointer dereference

Core was generated by `/usr/bin/connmand -n --nobacktrace --noplugin vpn'.
Program terminated with signal SIGSEGV, Segmentation fault.
>0  connman_device_get_last_connected_ident (device=0x0) at src/device.c:842
842		return device->last_connected_ident;
(gdb) bt
>0  connman_device_get_last_connected_ident (device=0x0) at src/device.c:842
>1  0x0098ec70 in calculate_score_last_connected (service=0x1ef9eb8) at src/service.c:7900
>2  calculate_score (service=0x1ef9eb8) at src/service.c:7994
>3  calculate_score (service=0x1ef9eb8) at src/service.c:7977
>4  0x0098eed8 in service_compare (a=0x1ef9eb8, b=0x1f010a0) at src/service.c:8138

Change-Id: I7e412f68f71472e28669941b9346ba73281ccd39
Signed-off-by: Nishant Chaprana <n.chaprana@samsung.com>