Fix crash on avrcp controller API (SIGSEGV)

Fix crash on avrcp controller API (SIGSEGV)

0  0xf722c048 in g_value_unset () from /lib/libgobject-2.0.so.0
1  0xaaeca726 in _bt_avrcp_control_set_property ()
2  0xaaece41c in __bt_bluez_request ()
3  0xaaed4d9c in ?? ()
4  0xf7432a60 in ?? () from /lib/libgio-2.0.so.0
5  0xf71338e8 in g_main_context_dispatch () from /lib/libglib-2.0.so.0
6  0xf7133c78 in ?? () from /lib/libglib-2.0.so.0
7  0xf7133fd8 in g_main_loop_run () from /lib/libglib-2.0.so.0
8  0xaae5726a in main ()

Change-Id: I8b5d77241c1b8788ba91531a4c80cd6f2f280970