domain: avoid potential memory leak in virDomainGraphicsListenSet*()

domain: avoid potential memory leak in virDomainGraphicsListenSet*()

virDomainGraphicsListenSetAddress() and
virDomainGraphicsListenSetNetwork() both set their respective char* to
NULL directly when asked to set it to NULL, which is okay as long as
it's already set to NULL. If these functions are ever called to clear
a listen object that has a valid string in address or network, it will
end up leaking the old value. Currently that doesn't happen, so this
is just a preemptive strike.