Fix Cycles allocating result too early

When tiled rendering was used the render result was
allocated at the end of every view layer render as
opposite of an intended end of all rendering.

Modify the render_result_end so that it only ensures
pixels are allocated if pixels are actually copied
over.

source/blender/render/intern/engine.c

@@ -435,8 +435,6 @@ void RE_engine_end_result(
     return;
   }
 
-  re_ensure_passes_allocated_thread_safe(re);
-
   if (re->engine && (re->engine->flag & RE_ENGINE_HIGHLIGHT_TILES)) {
     const HighlightedTile tile = highlighted_tile_from_result_get(re, result);
 
@@ -445,6 +443,7 @@ void RE_engine_end_result(
 
   if (!cancel || merge_results) {
     if (!(re->test_break(re->tbh) && (re->r.scemode & R_BUTS_PREVIEW))) {
+      re_ensure_passes_allocated_thread_safe(re);
       render_result_merge(re->result, result);
     }