[ENOMEM] fix access to unitialized memory.

If the serialize() call fails to write the object then we can't safely read varstore_prime fields. Fixes https://oss-fuzz.com/testcase-detail/5137462782066688.

src/hb-ot-layout-common.hh

@@ -2537,7 +2537,10 @@ struct VariationStore
 
     for (unsigned i = 0; i < inner_maps.length; i++)
       inner_maps[i].fini ();
-    return_trace (bool (varstore_prime->dataSets));
+
+    return_trace (
+        !c->serializer->in_error()
+        && varstore_prime->dataSets);
   }
 
   unsigned int get_region_index_count (unsigned int ivs) const