vnc: clear vs->tlscreds after unparenting it

This pointer should be cleared in vnc_display_close()
otherwise a use-after-free can happen when when using the
old style 'x509' and 'tls' options rather than a persistent
tls-creds -object, by issuing monitor commands to change
the vnc server like so:

Start with: -vnc unix:test.socket,x509,tls
Then use the following monitor command:
  change vnc unix:test.socket

After this the pointer is still set but invalid and a crash
can be triggered for instance by issuing the same command a
second time which will try to object_unparent() the same
pointer again.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
