Fix Segmentation Fault when UDID parsing failed by bailing out in case of error

Fixes #63.
diff --git a/src/libusbmuxd.c b/src/libusbmuxd.c
index 62af3e2..4a52f67 100644
--- a/src/libusbmuxd.c
+++ b/src/libusbmuxd.c
@@ -271,7 +271,7 @@
 	if (!devinfo->udid[0]) {
 		LIBUSBMUXD_ERROR("%s: Failed to get SerialNumber (UDID)!\n", __func__);
 		free(devinfo);
-		devinfo = NULL;
+		return NULL;
 	}
 	if (!devinfo->conn_type) {
 		LIBUSBMUXD_ERROR("%s: Failed to get ConnectionType!\n", __func__);