obexd: Fix crash while removing session

Crash occurs when removing a session with RemoveSession while another
session has been created but not yet registered.

Backtrace:
0  __strcmp_ssse3 () at ../sysdeps/i386/i686/multiarch/strcmp-ssse3.S:233
1  0xb758e7c3 in g_str_equal () from /lib/i386-linux-gnu/libglib-2.0.so.0
2  0x08073e56 in find_session (path=0x85c8504 "/org/bluez/obex/session0") at obexd/client/manager.c:146
3  remove_session (connection=0x85bc5e0, message=0x85bca98, user_data=0x0) at obexd/client/manager.c:216
4  0x08055f6f in process_message (connection=0x85bc5e0, message=<optimized out>, iface_user_data=0x0,
    method=<optimized out>, method=<optimized out>) at gdbus/object.c:285
5  0xb7672666 in ?? () from /lib/i386-linux-gnu/libdbus-1.so.3
6  0xb76624d7 in dbus_connection_dispatch () from /lib/i386-linux-gnu/libdbus-1.so.3
7  0x080532f8 in message_dispatch (data=0x85bc5e0) at gdbus/mainloop.c:76
8  0xb759f6bf in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
9  0xb759e9e3 in g_main_context_dispatch () from /lib/i386-linux-gnu/libglib-2.0.so.0
10 0xb759ed80 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
11 0xb759f1db in g_main_loop_run () from /lib/i386-linux-gnu/libglib-2.0.so.0
12 0x08052d74 in main (argc=1, argv=0xbfb344e4) at obexd/src/main.c:323
gdbus: Hold client reference during get name owner reply
unit: Use separate context for each D-Bus test case
tools: Add testing for powered, connectable and pairable commands
tools: Add a few more extra invalid parameters tests
tools: Add support for checking return value of commands
tools: Use second management channel to monitor events
tools: Add first set of discoverable management commands
tools: Fix tests for discoverable management commands
tools: Use connectable and powered setup stages for discoverable tests
core: Remove unneeded manager_init()
core: Rename adapter_init() to adapter_setup()
core: Introduce an adapter_cleanup() function
core: Introduce an adapter_find() function
core: Introduce adapter_find_by_id() function
audio: Prefer adapter_find functions over manager ones
health: Prefer adapter_find functions over manager ones
core: Prefer adapter_find functions over manager ones
core: Introduce an adapter_foreach() function
hostname: Prefer adapter_foreach over manager function