port-qmi: fix qmi_device_list_links usage

qmi_device_list_links can return TRUE, but with a NULL value for
the links list: check for validity before using the list.

src/mm-port-qmi.c

@@ -2089,7 +2089,11 @@ count_links_setup (MMPortQmi *self,
             return 0;
         }
 
+        if (links)
             return links->len;
+
+        /* No list of links returned, so there are none */
+        return 0;
     }
 
     if (self->priv->kernel_data_modes & MM_PORT_QMI_KERNEL_DATA_MODE_MUX_QMIWWAN)