Don't serialize null offsets in CPAL.

Fixes https://oss-fuzz.com/testcase-detail/5443213648330752

src/hb-ot-color-cpal-table.hh

@@ -83,7 +83,12 @@ struct CPALV1Tail
     auto *out = c->allocate_size<CPALV1Tail> (static_size);
     if (unlikely (!out)) return_trace (false);
 
+    out->paletteFlagsZ = 0;
+    if (paletteFlagsZ)
       out->paletteFlagsZ.serialize_copy (c, paletteFlagsZ, base, 0, hb_serialize_context_t::Head, palette_count);
+
+    out->paletteLabelsZ = 0;
+    if (paletteLabelsZ)
       out->paletteLabelsZ.serialize_copy (c, paletteLabelsZ, base, 0, hb_serialize_context_t::Head, palette_count);
 
     const hb_array_t<const NameID> colorLabels = (base+colorLabelsZ).as_array (color_count);