vnc: fix memleak of the "vnc-worker-output" name

vnc: fix memleak of the "vnc-worker-output" name

Fixes repeated memory leaks of 18 bytes when using VNC:

    Direct leak of 831024 byte(s) in 46168 object(s) allocated from:
        ...
        #4 0x7f6d2f919bdd in g_strdup_vprintf glib/gstrfuncs.c:514
        #5 0x56085cdcf660 in buffer_init util/buffer.c:59
        #6 0x56085ca6a7ec in vnc_async_encoding_start ui/vnc-jobs.c:177
        #7 0x56085ca6b815 in vnc_worker_thread_loop ui/vnc-jobs.c:240

Fixes: 543b95801f98 ("vnc: attach names to buffers")
Cc: Gerd Hoffmann <kraxel@redhat.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180807221830.3844-1-peter@lekensteyn.nl
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>