Fix NULL pointer dereference

Fix NULL pointer dereference

Thread 1 (LWP 13957):
>0  count_assoc_reject (key=0x0, value=0x0, user_data=0xbe0a3928) at src/service.c:843 --> connman (rpm)
>1  0xb6654674 in g_hash_table_foreach (hash_table=0x533200, func=0x4700ad <count_assoc_reject>, user_data=user_data@entry=0xbe0a3928) at ../glib/ghash.c:2056 --> libglib (rpm)
>2  0x0047bf9a in update_assoc_reject (service=0x4fb2a0, service=0x4fb2a0) at src/service.c:857 --> connman (rpm)
>3  __connman_service_update_from_network (network=network@entry=0x50d6a0) at src/service.c:11004 --> connman (rpm)
>4  0x0046a9f4 in connman_network_set_group (network=network@entry=0x50d6a0, group=group@entry=0x560ff8 "") at src/network.c:1587 --> connman (rpm)
>5  0x0044f12c in network_added (supplicant_network=0x51d430) at plugins/wifi.c:5031 --> connman (rpm)
>6  0x004516ba in callback_network_added (network=<optimized out>) at gsupplicant/supplicant.c:828 --> connman (rpm)
>7  scan_network_update (iter=<optimized out>, user_data=0x4f0380) at gsupplicant/supplicant.c:3790 --> connman (rpm)

Change-Id: I2f6e86c492dbc621b1c9f48506e8a89326568664
Signed-off-by: Jaehyun Kim <jeik01.kim@samsung.com>