Fix #122036: Preview image crash with asset browser

I can't reproduce this on release or debug builds. But from the strack
traces provided in a couple reports, it looks like the runtime pointer
is null. This makes sense because BKE_previewimg_blend_read is not
called here which is what allocates the runtime struct on file read.
To maintain a "valid" PreviewImage in more code and avoid the
complexity of making the runtime pointer optional, just allocate
it in BLO_blendhandle_get_preview_for_id.

source/blender/blenloader/intern/readblenentry.cc

@@ -270,8 +270,9 @@ PreviewImage *BLO_blendhandle_get_preview_for_id(BlendHandle *bh,
         if (preview_from_file == nullptr) {
           break;
         }
-
+        
         PreviewImage *result = static_cast<PreviewImage *>(MEM_dupallocN(preview_from_file));
+        result->runtime = MEM_new<blender::bke::PreviewImageRuntime>(__func__);
         bhead = blo_blendhandle_read_preview_rects(fd, bhead, result, preview_from_file);
         MEM_freeN(preview_from_file);
         return result;