spice: monitors_config: check pointer before dereferencing

Reported-by: NFabio Fantoni <fabio.fantoni@m2r.biz>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

ui/spice-display.c

@@ -549,6 +549,10 @@ static int interface_client_monitors_config(QXLInstance *sin,
     QemuUIInfo info;
     int rc;
 
+    if (!mc) {
+        return 1;
+    }
+
     /*
      * FIXME: multihead is tricky due to the way
      * spice has multihead implemented.