virnetdev: Resolve Coverity FORWARD_NULL

virnetdev: Resolve Coverity FORWARD_NULL

The complaint is that if cleanup is called when virFileReadAll fails,
then mcast->entries is NULL and could be dereferenced in the clear
function. After following the code some - I saw that the caller to
the function (virNetDevGetMulticastTable) will also call
virNetDevMcastListClear if this function returns -1, so this
isn't necessary, so I removed the call.