manager: return NULL for invalid ifindex in nm_manager_get_device_by_ifindex()

Internally, the device migth have negative or zero ifindex.
When calling nm_manager_get_device_by_ifindex(), the caller
wants to find a device with a valid ifindex, hence filter
out non-positive values.

(cherry picked from commit 31245cdd62)

src/nm-manager.c

@@ -1210,10 +1210,12 @@ nm_manager_get_device_by_ifindex (NMManager *self, int ifindex)
 	NMManagerPrivate *priv = NM_MANAGER_GET_PRIVATE (self);
 	NMDevice *device;
 
+	if (ifindex > 0) {
 		c_list_for_each_entry (device, &priv->devices_lst_head, devices_lst) {
 			if (nm_device_get_ifindex (device) == ifindex)
 				return device;
 		}
+	}
 
 	return NULL;
 }