device: check for NULL driver in add_port_driver

This patch fixes a crash in MMDevice::add_port_driver() due to
g_str_equal() dereferencing a NULL driver returned by
mm_device_utils_get_port_driver().

Bug reported on https://code.google.com/p/chromium/issues/detail?id=241823

src/mm-device.c

@@ -255,6 +255,8 @@ add_port_driver (MMDevice *self,
     guint i;
 
     driver = mm_device_utils_get_port_driver (udev_port);
+    if (!driver)
+        return;
 
     n_items = (self->priv->drivers ? g_strv_length (self->priv->drivers) : 0);
     if (n_items > 0) {