NSS: fix a use-after-free issue
    
    While handling well-known SIDs a debug statement tries to access memory that is
    already freed. This can be seen with the following output from valgrind.
    
    ==17600== Invalid read of size 4
    ==17600==    at 0x805ACC6: nss_cmd_getbysid (nsssrv_cmd.c:5458)
    ==17600==    by 0x805AF41: nss_cmd_getnamebysid (nsssrv_cmd.c:5509)
    ==17600==    by 0x80662F4: sss_cmd_execute (responder_cmd.c:161)
    ==17600==    by 0x8067015: client_cmd_execute (responder_common.c:249)
    ==17600==    by 0x80671F5: client_recv (responder_common.c:283)
    ==17600==    by 0x806741C: client_fd_handler (responder_common.c:335)
    ==17600==    by 0x45F5112: epoll_event_loop (tevent_epoll.c:728)
    ==17600==    by 0x45F5112: epoll_event_loop_once (tevent_epoll.c:926)
    ==17600==    by 0x45F32EE: std_event_loop_once (tevent_standard.c:114)
    ==17600==    by 0x45EF3BF: _tevent_loop_once (tevent.c:530)
    ==17600==    by 0x45EF5AB: tevent_common_loop_wait (tevent.c:634)
    ==17600==    by 0x45F326E: std_event_loop_wait (tevent_standard.c:140)
    ==17600==    by 0x45EF647: _tevent_loop_wait (tevent.c:653)
    ==17600==  Address 0x4b248a0 is 72 bytes inside a block of size 88 free'd
    ==17600==    at 0x402C26D: free (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
    ==17600==    by 0x45FEC9E: _talloc_free_internal (talloc.c:1057)
    ==17600==    by 0x45FEC9E: _talloc_free (talloc.c:1581)
    ==17600==    by 0x8066085: sss_cmd_done (responder_cmd.c:93)
    ==17600==    by 0x805A9B0: nss_check_well_known_sid (nsssrv_cmd.c:5382)
    ==17600==    by 0x805AC86: nss_cmd_getbysid (nsssrv_cmd.c:5455)
    ==17600==    by 0x805AF41: nss_cmd_getnamebysid (nsssrv_cmd.c:5509)
    ==17600==    by 0x80662F4: sss_cmd_execute (responder_cmd.c:161)
    ==17600==    by 0x8067015: client_cmd_execute (responder_common.c:249)
    ==17600==    by 0x80671F5: client_recv (responder_common.c:283)
    ==17600==    by 0x806741C: client_fd_handler (responder_common.c:335)
    ==17600==    by 0x45F5112: epoll_event_loop (tevent_epoll.c:728)
    ==17600==    by 0x45F5112: epoll_event_loop_once (tevent_epoll.c:926)
    ==17600==    by 0x45F32EE: std_event_loop_once (tevent_standard.c:114)
    ==17600==
    
    The patch contains a change to the unit tests which frees the memory in
    the wrapper for sss_cmd_done() too. This allows to detect this kind of
    issue in the unit tests as well.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>