[subset] Assert offsets are zero during relocation

If they're not, it's a bug in our subsetting logic somewhere.  So check.

src/hb-serialize.hh

@@ -294,12 +294,14 @@ struct hb_serialize_context_t
 	if (link.is_wide)
 	{
 	  auto &off = * ((BEInt<uint32_t, 4> *) (parent.head + link.position));
+	  assert (0 == off);
 	  off = offset;
 	  propagate_error (off == offset);
 	}
 	else
 	{
 	  auto &off = * ((BEInt<uint16_t, 2> *) (parent.head + link.position));
+	  assert (0 == off);
 	  off = offset;
 	  propagate_error (off == offset);
 	}