Check for existence of interface prior to setting terminate flag

https://bugzilla.redhat.com/show_bug.cgi?id=903480

During domain destruction it's possible that the learnIPAddressThread has
already removed the interface prior to the teardown filter path being run.
The teardown code would only be telling the thread to terminate.

src/nwfilter/nwfilter_learnipaddr.c

@@ -251,6 +251,14 @@ virNWFilterTerminateLearnReq(const char *ifname) {
     int ifindex;
     virNWFilterIPAddrLearnReqPtr req;
 
+    /* It's possible that it's already been removed as a result of
+     * virNWFilterDeregisterLearnReq during learnIPAddressThread() exit
+     */
+    if (virNetDevExists(ifname) != 1) {
+        virResetLastError();
+        return 0;
+    }
+
     if (virNetDevGetIndex(ifname, &ifindex) < 0) {
         virResetLastError();
         return rc;