telephony: Fixed memory leak

telephony: Fixed memory leak

Valgrind reported memory leak with below backtrace
==3471== 1,934 (80 direct, 1,854 indirect) bytes in 2 blocks are
definitely lost in loss record 190 of 194
==3471==    at 0x484801C: calloc (vg_replace_malloc.c:711)
==3471==    by 0x49D687B: dbus_malloc0 (dbus-memory.c:572)
==3471==    by 0x49BB9A3: _dbus_pending_call_new_unlocked (dbus-pending-call.c:124)
==3471==    by 0x4995963: dbus_connection_send_with_reply (dbus-connection.c:3449)
==3471==    by 0x135D8D: __dbus_request.constprop.8 (telephony.c:396)
==3471==    by 0x135E6D: __request_get_services (telephony.c:477)
==3471==    by 0x136F19: __add_modem (telephony.c:802)
==3471==    by 0x13732F: __response_get_modems (telephony.c:464)
==3471==    by 0x498FABB: complete_pending_call_and_unlock (dbus-connection.c:2340)
==3471==    by 0x49981BF: dbus_connection_dispatch (dbus-connection.c:4757)
==3471==    by 0x18E815: message_dispatch (mainloop.c:72)
==3471==    by 0x48BB8E7: g_main_dispatch (gmain.c:3234)
==3471==    by 0x48BB8E7: g_main_context_dispatch (gmain.c:3887

When dbus response is received then we should unref memory allocated by
libdbus

Change-Id: I81c514506f0df42e537d63382b2eb2ebc51c58c8
Signed-off-by: Saurav Babu <saurav.babu@samsung.com>