[subset] use create_sub_blob instead of manually copying the post table.

src/hb-ot-post-table.hh

@@ -84,26 +84,20 @@ struct post
 
   inline bool subset (hb_subset_plan_t *plan) const
   {
+    unsigned int post_prime_length;
     hb_blob_t *post_blob = OT::Sanitizer<post>().sanitize (hb_face_reference_table (plan->source, HB_OT_TAG_post));
-    post *post_table = (post *) hb_blob_get_data (post_blob, nullptr);
+    hb_blob_t *post_prime_blob = hb_blob_create_sub_blob (post_blob, 0, post::static_size);
+    post *post_prime = (post *) hb_blob_get_data_writable (post_prime_blob, &post_prime_length);
+    hb_blob_destroy (post_blob);
 
-    post *post_prime = (post *) malloc (post::static_size);
-    if (unlikely (!post_prime))
+    if (unlikely (!post_prime || post_prime_length != post::static_size))
     {
-      hb_blob_destroy (post_blob);
-      DEBUG_MSG(SUBSET, nullptr, "Unable to alloc %lu for post subset output.", (unsigned long) post::static_size);
+      hb_blob_destroy (post_prime_blob);
+      DEBUG_MSG(SUBSET, nullptr, "Invalid source post table with length %d.", post_prime_length);
       return false;
     }
 
-    memcpy (post_prime, post_table, post::static_size);
-    hb_blob_destroy (post_blob);
-
     post_prime->version.major.set (3); // Version 3 does not have any glyph names.
-    hb_blob_t *post_prime_blob = hb_blob_create ((const char *) post_prime,
-						 post::static_size,
-						 HB_MEMORY_MODE_READONLY,
-						 post_prime,
-						 free);
     bool result = hb_subset_plan_add_table (plan, HB_OT_TAG_post, post_prime_blob);
     hb_blob_destroy (post_prime_blob);