hw/net/can: Fix segfaults when using the devices without bus

The CAN devices can currently be used to crash QEMU, e.g.:

$ x86_64-softmmu/qemu-system-x86_64 -device kvaser_pci
Segmentation fault (core dumped)

So we've got to add a proper check here that the corresponding
bus is available.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-Id: <1521193892-15552-2-git-send-email-thuth@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/net/can/can_sja1000.c

@@ -866,6 +866,10 @@ int can_sja_connect_to_bus(CanSJA1000State *s, CanBusState *bus)
 {
     s->bus_client.info = &can_sja_bus_client_info;
 
+    if (!bus) {
+        return -EINVAL;
+    }
+
     if (can_bus_insert_client(bus, &s->bus_client) < 0) {
         return -1;
     }