[blob] Fix-up recent mistake in hb_blob_create() destroy(user_data)

https://github.com/harfbuzz/harfbuzz/pull/3026#discussion_r653437491

src/hb-blob.cc

diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index 4924eba..4939bf2 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -73,7 +73,11 @@
 		hb_destroy_func_t  destroy)
 {
   if (!length)
+  {
+    if (destroy)
+      destroy (user_data);
     return hb_blob_get_empty ();
+  }
 
   hb_blob_t *blob = hb_blob_create_or_fail (data, length, mode,
 					    user_data, destroy);