conf: net: Fix deadlock if assignment of network def fails

When the assignment fails, the network object is not unlocked and next
call that would use it deadlocks.
(cherry picked from commit f8230891243f86e920d04a0751512cc31055ff8c)

src/conf/network_conf.c

@@ -295,6 +295,7 @@ virNetworkAssignDef(virNetworkObjListPtr nets,
 
     if ((network = virNetworkFindByName(nets, def->name))) {
         if (virNetworkObjAssignDef(network, def, live) < 0) {
+            virNetworkObjUnlock(network);
             return NULL;
         }
         return network;