network: Set to NULL after virNetworkDefFree()

network: Set to NULL after virNetworkDefFree()

which frees all allocated memory but doesn't set the passed pointer to
NULL.  Therefore, we must do it ourselves. This is causing actual
libvirtd crash: Basically, when doing 'virsh net-edit' the newDef should
be dropped.  And the memory is freed, indeed. However, the pointer is
not set to NULL but kept instead. And the next duo of calls 'virsh
net-start' and 'virsh net-destroy' starts the disaster. The latter one
does the same as 'virsh destroy'; it sees that newDef is nonNULL so it
replaces def with newDef (which has been freed already as said a few
lines above). Therefore any subsequent call accessing def will hit the ground.