[subset] fail on offset overflow in tables that we don't repack.

Fixes: https://oss-fuzz.com/testcase-detail/5229304507138048

src/hb-subset.cc

@@ -73,7 +73,10 @@ _repack (hb_tag_t tag, const hb_serialize_context_t& c)
 {
   if (tag != HB_OT_TAG_GPOS
       &&  tag != HB_OT_TAG_GSUB)
-    return c.copy_blob ();
+  {
+    // Check for overflow in a non-handled table.
+    return c.successful () ? c.copy_blob () : nullptr;
+  }
 
   if (!c.offset_overflow ())
     return c.copy_blob ();