Libvirt: null pointer dereference in udevconnectlistallinterfaces()

A NULL pointer dereference flaw was found in the udevConnectListAllInterfaces() function in libvirt. This issue can occur when detaching a host interface while at the same time collecting the list of interfaces via virConnectListAllInterfaces API. This flaw could be used to perform a denial of service attack by causing the libvirt daemon to crash.