tools/obexctl: Fix retrieving non-existent iterator

tools/obexctl: Fix retrieving non-existent iterator

If transfer is completed with one time file transfer,
transfer_property_changed() is called with the iter argument, which is
NULL, as below:

  #0  transfer_property_changed (proxy=<optimized out>, name=<optimized out>, iter=0x0, user_data=0x5555557b9060) at tools/obexctl.c:1896
  #1  0x000055555556e3cf in properties_changed (conn=<optimized out>, msg=<optimized out>, user_data=0x5555557c8300) at gdbus/client.c:433
  #2  0x000055555556c830 in signal_filter (connection=0x5555557c65a0, message=0x5555557ccea0, user_data=0x5555557cdbf0) at gdbus/watch.c:407
  #3  0x000055555556c291 in message_filter (connection=0x5555557c65a0, message=0x5555557ccea0, user_data=<optimized out>) at gdbus/watch.c:557
  #4  0x00007ffff7888661 in dbus_connection_dispatch () from /lib/x86_64-linux-gnu/libdbus-1.so.3
  #5  0x000055555556b010 in message_dispatch (data=0x5555557c65a0) at gdbus/mainloop.c:72
  #6  0x00007ffff7b0be25 in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
  #7  0x00007ffff7b0c1f0 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
  #8  0x00007ffff7b0c502 in g_main_loop_run () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
  #9  0x0000555555577594 in bt_shell_run () at src/shared/shell.c:833
  #10 0x000055555556516d in main (argc=<optimized out>, argv=<optimized out>) at tools/obexctl.c:2116

Change-Id: Ia570e8c9f140cf9795b79a9d1cc836ff174ea2c9
Signed-off-by: Amit Purwar <amit.purwar@samsung.com>