ui/vnc: fix leak of SocketAddress **

Extract the (correct) cleaning code as a new function vnc_free_addresses() then
use it to remove the memory leaks.

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>