Fix the crash

Fix the crash

gdb) bt
0  0x00520b98 in ext_prop_read_cb (success=false, att_ecode=0 '\000', value=0x0, length=0, user_data=0xad5310) at src/shared/gatt-client.c:746
1  0x0051e62a in read_cb (opcode=<optimized out>, pdu=<optimized out>, length=<optimized out>, user_data=<optimized out>) at src/shared/gatt-client.c:2418
2  0x0051d5a6 in disc_att_send_op (data=0xad5cd0) at src/shared/att.c:569
3  0x0051dffc in disconnect_cb (io=<optimized out>, user_data=0xadb400) at src/shared/att.c:602
4  0x00524298 in watch_callback (channel=<optimized out>, cond=<optimized out>, user_data=<optimized out>) at src/shared/io-glib.c:170
5  0xf75318e8 in g_main_dispatch (context=0xac5f80) at gmain.c:3234
6  g_main_context_dispatch (context=context@entry=0xac5f80) at gmain.c:3887
7  0xf7531c78 in g_main_context_iterate (context=0xac5f80, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at gmain.c:3960
8  0xf7531fd8 in g_main_loop_run (loop=0xac5ee0) at gmain.c:4156
9  0x004b032c in main (argc=<optimized out>, argv=<optimized out>) at src/main.c:723
gdb) bt full 3
0  0x00520b98 in ext_prop_read_cb (success=false, att_ecode=0 '\000', value=0x0, length=0, user_data=0xad5310) at src/shared/gatt-client.c:746
op = 0xad5310
client = <optimized out>
discovering = 247
desc_attr = <optimized out>
1  0x0051e62a in read_cb (opcode=<optimized out>, pdu=<optimized out>, length=<optimized out>, user_data=<optimized out>) at src/shared/gatt-client.c:2418
req = <optimized out>
op = <optimized out>
success = <optimized out>
att_ecode = <optimized out>
value = 0x0
value_len = <optimized out>
2  0x0051d5a6 in disc_att_send_op (data=0xad5cd0) at src/shared/att.c:569
op = 0xad5cd0

Change-Id: I6f817e21a4a75c3c36f62d1d102e189be5088968