networkBandwidthUpdate: Don't blindly dereference pointers

It may happen that an interface don't have any bandwidth set and
a new one is to be set. In that case, @ifaceBand will be NULL.
This will cause troubles later in the code when deciding what to
do.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/network/bridge_driver.c

@@ -5048,8 +5048,8 @@ networkBandwidthUpdate(virDomainNetDefPtr iface,
 
     /* Okay, there are three possible scenarios: */
 
-    if (ifaceBand->in && ifaceBand->in->floor &&
-        newBandwidth->in && newBandwidth->in->floor) {
+    if (ifaceBand && ifaceBand->in && ifaceBand->in->floor &&
+        newBandwidth && newBandwidth->in && newBandwidth->in->floor) {
         /* Either we just need to update @floor .. */
 
         if (virNetDevBandwidthUpdateRate(network->def->bridge,