rpc: Fix possible crash when MDNSAddEntry fails

If virNetServerMDNSAddEntry() fails when adding a service to a server,
it doesn't decrease the number of services.  Hence access to their
members segfaults (e.g. when free()-ing the sruct).

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

src/rpc/virnetserver.c

@@ -974,8 +974,10 @@ int virNetServerAddService(virNetServerPtr srv,
 
         if (!virNetServerMDNSAddEntry(srv->mdnsGroup,
                                       mdnsEntryName,
-                                      port))
+                                      port)) {
+            srv->nservices--;
             goto error;
+        }
     }
 
     srv->services[srv->nservices-1] = svc;