Use ref counting for network during SignalPoll method call of wpa_supplicant.

Use ref counting for network during SignalPoll method call of wpa_supplicant.

Description: This patch fixes the issue in which dangling network pointer
is present as user_data of signalpoll_callback.

This dangling pointer crashes connman with below backtrace:-

>>> bt
\#0  connman_device_get_ident (device=0x6e6f632f) at src/device.c:592
\#1  0xb6f54994 in __connman_network_get_ident (network=network@entry=0xb87715b0) at src/network.c:1560
\#2  0xb6f652da in connman_service_lookup_from_network (network=network@entry=0xb87715b0) at src/service.c:10177
\#3  0xb6f655ee in __connman_service_notify_strength_changed (network=network@entry=0xb87715b0) at src/service.c:10523
\#4  0xb6f5527c in connman_network_set_strength (network=network@entry=0xb87715b0, strength=strength@entry=69 'E') at src/network.c:2704
\#5  0xb6f36ac0 in signalpoll_callback (result=<optimized out>, maxspeed=39, strength=69, user_data=0xb87715b0) at plugins/wifi.c:3866
\#6  0xb6f3b68e in interface_signalpoll_result (error=<optimized out>, iter=<optimized out>, user_data=0xb8782af8) at gsupplicant/supplicant.c:6348
\#7  0xb6f4335a in method_call_reply (call=0xb8771ec0, user_data=0xb8782c98) at gsupplicant/dbus.c:476
\#8  0xb6da23a4 in ?? () from /lib/libdbus-1.so.3
\#9  0xb6da5fa0 in dbus_connection_dispatch () from /lib/libdbus-1.so.3
\#10 0xb6f94dce in message_dispatch (data=0xb875fe78) at gdbus/mainloop.c:72
\#11 0xb6e21d84 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
\#12 0xb6e22008 in ?? () from /lib/libglib-2.0.so.0
\#13 0xb6e22268 in g_main_loop_run () from /lib/libglib-2.0.so.0
\#14 0xb6f29d3e in main (argc=<optimized out>, argv=<optimized out>) at src/main.c:1373

Change-Id: Ia171c2ddabf6a4f9c3d6a6bbd3763398b6e0ce46
Signed-off-by: Nishant Chaprana <n.chaprana@samsung.com>