Fix: Crash updating draw cache of original curves data

Error in c67e5628d2 which mistook clearing
the pointer for clearing the values in the struct.

source/blender/draw/intern/draw_cache_impl_curves.cc

@@ -78,7 +78,7 @@ static void curves_batch_cache_init(Curves &curves)
     curves.batch_cache = cache;
   }
   else {
-    cache = {};
+    cache->curves_cache = {};
   }
 
   cache->is_dirty = false;