UrfOfonoManager: check NULL proxy (LP: #1381818)

If ofono disappears from the bus, ofono->proxy is
unref'd and set to NULL.  This change adds a check
in get_modems_cb() to prevent a crash caused by
passing this NULL proxy to g_dbus_proxy_call_finish().
Merge pull request #17 from tonyespy/lp1381818

UrfOfonoManager: check NULL proxy (LP: #1381818)