PROXY: Fix use after free
    
    The dbus_req and associated talloc context are no longer valid after
    execution of the function sbus_request_return_and_finish even if error code
    was returned.
    
    ==32479== Invalid read of size 8
    ==32479==    at 0x131F275F: client_registration (proxy_init.c:474)
    ==32479==    by 0x529709E: sbus_request_invoke_or_finish (sssd_dbus_request.c:69)
    ==32479==    by 0x52949B3: sbus_handler_got_caller_id (sssd_dbus_connection.c:555)
    ==32479==    by 0x89B27E3: tevent_common_loop_immediate (tevent_immediate.c:135)
    ==32479==    by 0x89B70CD: epoll_event_loop_once (tevent_epoll.c:907)
    ==32479==    by 0x89B57D6: std_event_loop_once (tevent_standard.c:114)
    ==32479==    by 0x89B1FBC: _tevent_loop_once (tevent.c:530)
    ==32479==    by 0x89B215A: tevent_common_loop_wait (tevent.c:634)
    ==32479==    by 0x89B5776: std_event_loop_wait (tevent_standard.c:140)
    ==32479==    by 0x529E255: server_loop (server.c:668)
    ==32479==    by 0x40DBC5: main (data_provider_be.c:2915)
    ==32479==  Address 0xb700858 is 104 bytes inside a block of size 136 free'd
    ==32479==    at 0x4C2AD17: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==32479==    by 0x8BBE462: _talloc_free (in /usr/lib64/libtalloc.so.2.1.1)
    ==32479==    by 0x52971A4: sbus_request_finish (sssd_dbus_request.c:95)
    ==32479==    by 0x529731A: sbus_request_return_and_finish (sssd_dbus_request.c:119)
    ==32479==    by 0x131F264D: client_registration (proxy_init.c:443)
    ==32479==    by 0x529709E: sbus_request_invoke_or_finish (sssd_dbus_request.c:69)
    ==32479==    by 0x52949B3: sbus_handler_got_caller_id (sssd_dbus_connection.c:555)
    ==32479==    by 0x89B27E3: tevent_common_loop_immediate (tevent_immediate.c:135)
    ==32479==    by 0x89B70CD: epoll_event_loop_once (tevent_epoll.c:907)
    ==32479==    by 0x89B57D6: std_event_loop_once (tevent_standard.c:114)
    ==32479==    by 0x89B1FBC: _tevent_loop_once (tevent.c:530)
    ==32479==    by 0x89B215A: tevent_common_loop_wait (tevent.c:634)
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2573
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>