Allow get_table() to return NULL

src/hb-ft.c

diff --git a/src/hb-ft.c b/src/hb-ft.c
index b5dd657..3e91eff 100644
--- a/src/hb-ft.c
+++ b/src/hb-ft.c
@@ -159,7 +159,7 @@
   /* TODO Use FT_Memory? */
   buffer = malloc (length);
   if (buffer == NULL)
-    return hb_blob_create_empty ();
+    return NULL;
 
   error = FT_Load_Sfnt_Table (ft_face, tag, 0, buffer, &length);
   if (error)