urfkill crashed with SIGSEGV in g_bit_lock()

I ran into this crash while playing around with ofono restarts on krillin ( rtm image #106 ). Restarting ofono with the wrong device/number of SIM slots will trigger this crash:

Here's the backtrace from gdb:

(urfkilld:9481): GLib-GIO-CRITICAL **: g_dbus_proxy_call_finish_internal: assertion 'G_IS_DBUS_PROXY (proxy)' failed

(urfkilld:9481): GLib-CRITICAL **: g_variant_get_type_string: assertion 'value != NULL' failed

Program received signal SIGSEGV, Segmentation fault.
0xb6d938be in g_bit_lock () from /lib/arm-linux-gnueabihf/libglib-2.0.so.0
(gdb) bt
#0 0xb6d938be in g_bit_lock () from /lib/arm-linux-gnueabihf/libglib-2.0.so.0
#1 0xb6ddb8c6 in g_variant_n_children ()
   from /lib/arm-linux-gnueabihf/libglib-2.0.so.0
#2 0xb6ddb904 in g_variant_get_child_value ()
   from /lib/arm-linux-gnueabihf/libglib-2.0.so.0
#3 0x00019794 in ofono_get_modems_cb (source_object=<optimized out>,
    res=<optimized out>, user_data=0x3ce80) at urf-ofono-manager.c:170
#4 0xb6eeb970 in g_simple_async_result_complete ()
   from /usr/lib/arm-linux-gnueabihf/libgio-2.0.so.0
#5 0xb6f34b7e in ?? () from /usr/lib/arm-linux-gnueabihf/libgio-2.0.so.0
Backtrace stopped: previous frame identical to this frame (corrupt stack?)

This could be triggered by the fact that the proxy becomes invalid before the callback is made from the main thread loop. A simple validity check to ensure that the proxy is still valid before calling the finish call would probably solve this.

Steps to reproduce:

As root, run restart ofono on krillin without specifying the device or number of SIM slots.

Marking this as High, as this is a contrived scenario as normally the OFONO_RIL_DEVICE and OFONO_RIL_NUM_SIM_SLOTS should be set when calling 'start' or 'restart' ofono from the command-line.